Process-Sensitive Metrology Systems and Methods

ABSTRACT

A lithography system includes an illumination source and a set of projection optics. The illumination source directs a beam of illumination from an off-axis illumination pole to a pattern mask. The pattern mask includes a set of pattern elements to generate a set of diffracted beams including illumination from the illumination pole. At least two diffracted beams of the set of diffracted beams received by the set of projection optics are asymmetrically distributed in a pupil plane of the set of projection optics. The at least two diffracted beams of the set of diffracted beams are asymmetrically incident on the sample to form a set of fabricated elements corresponding to an image of the set of pattern elements. The set of fabricated elements on the sample includes one or more indicators of a location of the sample along an optical axis of the set of projection optics.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Application Ser. No. 62/205,410, filed Aug. 14, 2015,entitled LITHOGRAPHY-AWARE FOCUS/DOSE MONITORING TARGET DESIGN METHOD,naming Myungjun Lee, Mark D. Smith, Sanjay Kapasi, Stillian Pandev, andDimitry Sanko as inventors, which is incorporated herein by reference inthe entirety.

The present application claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Application Ser. No. 62/205,529, filed Aug. 14, 2015,entitled HIGHLY SENSITIVE AND COST-EFFECTIVE FOCUS MONITORING TECHNIQUESUSING THE BINARY MASK WITH THE OPTIMIZED OFF-AXIS ILLUMINATION, namingMyungjun Lee AND Mark D. Smith as inventors, which is incorporatedherein by reference in the entirety.

The present application claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Application Ser. No. 62/297,697, filed Feb. 19, 2016,entitled HIGHLY SENSITIVE FOCUS MONITORING TECHNIQUE BASED ONILLUMINATION AND TARGET CO-OPTIMIZATION, naming Myungjun Lee, Mark D.Smith, Pradeep Subrahmanyan, and Ady Levy as inventors, which isincorporated herein by reference in the entirety.

TECHNICAL FIELD

The present disclosure relates generally to metrology, and moreparticularly, to co-optimization of illumination sources and metrologytargets with process-sensitive pattern masks.

BACKGROUND

Semiconductor lithography tools must typically operate within tighttolerances to properly write features having narrow linewidths and highdensities. For example, process parameters such as the focal position ofthe sample and the dose of illumination received by the sample may beaccurately monitored to ensure that printed features are within thedesired specifications. Process-sensitive metrology targets arespecialized marks patterned onto the wafer during a lithography step inwhich one or more characteristics of the metrology targets (e.g.alignment of two features) are indicative of a value of a processparameter associated with the lithography step. A process-sensitivemetrology target is typically generated as images of pattern masksgenerated by the lithography tool and may be influenced by particularfeatures on the pattern mask or the illumination source. Further, it isdesirable that process-sensitive pattern masks to be used in asemiconductor fabrication line be cost-effective and integrate with thepattern masks used to write the semiconductor devices under production.Therefore, it would be desirable to provide a system and method forcuring defects such as those identified above.

SUMMARY

A lithography system is disclosed, in accordance with one or moreillustrative embodiments of the present disclosure. In one illustrativeembodiment, the system includes an illumination source configured todirect a beam of illumination from an off-axis illumination pole to apattern mask. In another illustrative embodiment, the pattern maskincludes a set of pattern elements configured to generate a set ofdiffracted beams including illumination from the illumination pole. Inanother illustrative embodiment, the system includes a set of projectionoptics. In another illustrative embodiment, at least two diffractedbeams of the set of diffracted beams received by the set of projectionoptics are asymmetrically distributed in a pupil plane of the set ofprojection optics. In another illustrative embodiment, the at least twodiffracted beams of the set of diffracted beams are asymmetricallyincident on the sample to form a set of fabricated elementscorresponding to an image of the set of pattern elements. In anotherillustrative embodiment, the set of fabricated elements on the sampleincludes one or more indicators of a location of the sample along anoptical axis of the set of projection optics.

A lithography system is disclosed in accordance with one or moreillustrative embodiments of the present disclosure. In one illustrativeembodiment, the system includes an off-axis illumination source. Inanother illustrative embodiment, the illumination source includes afirst illumination pole and a second illumination pole. In anotherillustrative embodiment, the first and second illumination poles aresymmetrically distributed with respect to an optical axis. In anotherillustrative embodiment, the off-axis illumination source is configuredto direct illumination from the first and second illumination poles to apattern mask. In another illustrative embodiment, the pattern maskincludes a set of pattern elements. In another illustrative embodiment,the set of pattern elements is configured to generate a first set ofdiffracted beams including illumination from a first illumination polediffracted from the set of pattern elements. In another illustrativeembodiment, the set of pattern elements is configured to generate asecond set of diffracted beams including illumination from the secondillumination pole. In another illustrative embodiment, the systemincludes a set of projection optics. In another illustrative embodiment,at least two diffracted beams of the first set of diffracted beamsreceived by the set of projection optics are symmetrically distributedin a pupil plane of the set of projection optics. In anotherillustrative embodiment, at least two diffracted beams of the second setof diffracted beams received by the set of projection optics overlap thefirst set of diffracted beams in the pupil plane. In anotherillustrative embodiment, the at least two diffracted beams of the firstand second sets of diffracted beams of the set of diffracted beams forma set of fabricated elements on the sample corresponding to an image ofthe set of pattern elements. In another illustrative embodiment, the setof fabricated elements on the sample includes one or more indicators ofa dose of illumination on the sample associated with at least twodiffracted beams of the first and second sets of diffracted beams.

A metrology system is disclosed in accordance with one or moreillustrative embodiments of the present disclosure. In one illustrativeembodiment, the system includes a sample stage configured to support asubstrate with a metrology target disposed upon the substrate. Inanother illustrative embodiment, the metrology target is associated withan image of a pattern mask generated by a lithography system. In anotherillustrative embodiment, the pattern mask includes a set of patternelements configured to generate a set of diffracted beams includingillumination from an off-axis illumination pole of the lithographysystem. In another illustrative embodiment, at least two diffractedbeams of the set of diffracted beams received by the lithography systemare asymmetrically distributed in a pupil plane of the lithographysystem. In another illustrative embodiment, the at least two diffractedbeams of the set of diffracted beams are asymmetrically incident on thesample to form a set of fabricated elements of the metrology target. Inanother illustrative embodiment, the set of fabricated elements of themetrology target includes one or more indicators of a location of thesample along an optical axis of the set of projection optics of thelithography system. In another illustrative embodiment, the systemincludes at least one illumination source configured to illuminate themetrology target. In another illustrative embodiment, the systemincludes at least one detector configured to receive illumination fromthe metrology target. In another illustrative embodiment, the systemincludes at least one controller communicatively coupled to the detectorand configured to determine the location of the sample along the opticalaxis of the set of projection optics based on the one or moreindicators.

A metrology system is disclosed, in accordance with one or moreillustrative embodiments of the present disclosure. In one illustrativeembodiment, the system includes a sample stage configured to support asubstrate with a metrology target disposed upon the substrate. Inanother illustrative embodiment, the metrology target is associated withan image of a pattern mask generated by a lithography system. In anotherillustrative embodiment, the pattern mask includes a set of patternelements configured to generate a set of diffracted beams includingillumination from a first illumination pole and a second illuminationpole of the lithography system. In another illustrative embodiment, thefirst and second illumination poles of the lithography system aresymmetrically distributed with respect to an optical axis of thelithography system. In another illustrative embodiment, at least twodiffracted beams of the first set of diffracted beams received by thelithography system are symmetrically distributed in a pupil plane of thelithography system. In another illustrative embodiment, at least twodiffracted beams of the second set of diffracted beams received by theset of projection optics overlap the first set of diffracted beams inthe pupil plane of the lithography system. In another illustrativeembodiment, the at least two diffracted beams of the first and secondsets of diffracted beams of the set of diffracted beams aresymmetrically incident on the sample to form a set of fabricatedelements of the metrology target. In another illustrative embodiment,the set of fabricated elements of the metrology target includes one ormore indicators of a dose of illumination on the sample associated withat least two diffracted beams of the first and second sets of diffractedbeams. In another illustrative embodiment, the system includes at leastone illumination source configured to illuminate the metrology target.In another illustrative embodiment, the system includes at least onedetector configured to receive illumination from the metrology target.In another illustrative embodiment, the system includes at least onecontroller communicatively coupled to the detector and configured todetermine the dose of illumination on the metrology target associatedwith the at least two diffracted beams of the first and second sets ofdiffracted beams based on the one or more indicators.

A method for determining a position of a sample along an optical axis ofa lithography system is disclosed in accordance with one or moreillustrative embodiments of the present disclosure. In one illustrativeembodiment, the method includes generating an image of a pattern maskwith a lithography system including an off-axis illumination pole. Inanother illustrative embodiment, the pattern mask includes a set ofpattern elements configured to generate a set of diffracted beamsincluding illumination from an off-axis illumination pole of thelithography system. In another illustrative embodiment, at least twodiffracted beams of the set of diffracted beams received by thelithography system are asymmetrically distributed in a pupil plane ofthe lithography system. In another illustrative embodiment, the at leasttwo diffracted beams of the set of diffracted beams are asymmetricallyincident on the sample to form a set of fabricated elements of themetrology target. In another illustrative embodiment, the set offabricated elements of the metrology target includes one or moreindicators of a location of the sample along an optical axis of the setof projection optics of the lithography system. In another illustrativeembodiment, the method includes measuring the one or more indicators ofthe location of the sample along the optical axis of the set ofprojection optics of the lithography system using a metrology system. Inanother illustrative embodiment, the method includes determining thelocation of the sample along the optical axis of the set of projectionoptics based on the one or more indicators.

A method for determining a dose of illumination in a lithography systemis disclosed in accordance with one or more illustrative embodiments ofthe present disclosure. In one illustrative embodiment, the methodincludes generating an image of a pattern mask with a lithography systemincluding an off-axis illumination pole. In another illustrativeembodiment, the pattern mask includes a set of pattern elementsconfigured to generate a set of diffracted beams including illuminationfrom a first illumination pole and a second illumination pole of thelithography system. In another illustrative embodiment, the first andsecond illumination poles of the lithography system are symmetricallydistributed with respect to an optical axis of the lithography system.In another illustrative embodiment, at least two diffracted beams of thefirst set of diffracted beams received by the lithography system aresymmetrically distributed in a pupil plane of the lithography system. Inanother illustrative embodiment, at least two diffracted beams of thesecond set of diffracted beams received by the set of projection opticsoverlap the first set of diffracted beams in the pupil plane of thelithography system. In another illustrative embodiment, the at least twodiffracted beams of the first and second sets of diffracted beams of theset of diffracted beams are symmetrically incident on the sample to forma set of fabricated elements of the metrology target. In anotherillustrative embodiment, the set of fabricated elements of the metrologytarget includes one or more indicators of a dose of illumination on thesample associated with at least two diffracted beams of the first andsecond sets of diffracted beams. In another illustrative embodiment, themethod includes measuring the one or more indicators of the dose ofillumination on the metrology target associated with at least twodiffracted beams of the first and second sets of diffracted beams. Inanother illustrative embodiment, the method includes determining thedose of illumination on the metrology target based on the one or moreindicators.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not necessarily restrictive of the invention as claimed. Theaccompanying drawings, which are incorporated in and constitute a partof the specification, illustrate embodiments of the invention andtogether with the general description, serve to explain the principlesof the invention.

BRIEF DESCRIPTION OF DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1A is a conceptual view illustrating a system including alithography sub-system for lithographically printing one or morepatterns to a sample, in accordance with one or more embodiments of thepresent disclosure.

FIG. 1B is a conceptual view illustrating a metrology sub-system, inaccordance with one or more embodiments of the present disclosure.

FIG. 1C is a conceptual view illustrating a metrology sub-system, inaccordance with one or more embodiments of the present disclosure.

FIG. 2 is a schematic view illustrating a pattern mask including asegmented pattern element, in accordance with one or more embodiments ofthe present disclosure.

FIG. 3A is a conceptual view of the lithography sub-system illustratingmultiple diffracted beams generated by a pattern mask, in accordancewith one or more embodiments of the present disclosure.

FIG. 3B is a conceptual view of a pupil plane of the set of projectionoptics illustrating the relative positions of diffracted beams withinthe pupil plane, in accordance with one or more embodiments of thepresent disclosure.

FIG. 3C is a plot illustrating the relative acid concentration within aresist layer of a sample exposed by asymmetric illumination as shown inFIGS. 3A and 3B, in accordance with one or more embodiments of thepresent disclosure.

FIG. 3D is a conceptual view of the lithography sub-system illustratingmultiple diffracted beams generated by a pattern mask, in accordancewith one or more embodiments of the present disclosure.

FIG. 3E is a conceptual view of a pupil plane of the set of projectionoptics illustrating the relative positions of diffracted beams withinthe pupil plane, in accordance with one or more embodiments of thepresent disclosure.

FIG. 3F is a plot illustrating the relative acid concentration within aresist layer of a sample exposed by asymmetric illumination as shown inFIGS. 3D and 3E, in accordance with one or more embodiments of thepresent disclosure.

FIG. 3G is a conceptual view of the lithography sub-system illustratingmultiple diffracted beams generated by a pattern mask, in accordancewith one or more embodiments of the present disclosure.

FIG. 3H is a conceptual view of a pupil plane of the set of projectionoptics illustrating the relative positions of symmetric diffracted beamswithin the pupil plane, in accordance with one or more embodiments ofthe present disclosure.

FIG. 3I is a plot illustrating the relative acid concentration within aresist layer 116 of sample exposed by symmetric illumination as shown inFIGS. 3D and 3E, in accordance with one or more embodiments of thepresent disclosure.

FIG. 4A is a plot illustrating the distribution of diffracted beams in apupil plane associated with an off-axis single-pole illumination sourceand a pattern mask with a pitch of 80 nm, in accordance with one or moreembodiments of the present disclosure.

FIG. 4B is a plot illustrating the distribution of diffracted beams in apupil plane associated with an off-axis single-pole illumination sourceand a pattern mask with a pitch of 100 nm, in accordance with one ormore embodiments of the present disclosure.

FIG. 4C is a plot illustrating the distribution of diffracted beams in apupil plane associated with an off-axis single-pole illumination sourceand a pattern mask with a pitch of 140 nm, in accordance with one ormore embodiments of the present disclosure.

FIG. 4D is a plot illustrating the distribution of diffracted beams in apupil plane associated with an off-axis single-pole illumination sourceand a pattern mask with a pitch of 150 nm, in accordance with one ormore embodiments of the present disclosure.

FIG. 5 is a schematic view of simulated printed pattern profiles forpattern masks having pitch values ranging from 80 nm to 160 nm, inaccordance with one or more embodiments of the present disclosure.

FIG. 6 is a plot illustrating pattern placement error (PPE) associatedwith a measurement of the deviation of the top of the printed patternsfor pitch values ranging from 75 nm to 140 nm, in accordance with one ormore embodiments of the present disclosure.

FIG. 7A is a top view of a focus-sensitive pattern mask includingfocus-sensitive pattern elements and focus-insensitive pattern elements,in accordance with one or more embodiments of the present disclosure.

FIG. 7B is an enlarged view of a portion of a focus-sensitive patternelement, in accordance with one or more embodiments of the presentdisclosure.

FIG. 7C is an enlarged view of a portion of a focus-insensitive patternelement, in accordance with one or more embodiments of the presentdisclosure.

FIG. 8A is a top view of a focus-sensitive printed metrology targetcorresponding to a focus-sensitive pattern mask, in accordance with oneor more embodiments of the present disclosure.

FIG. 8B is a plot illustrating an exemplary relationship between thefocal position of a sample and a misalignment metric, in accordance withone or more embodiments of the present disclosure.

FIG. 9A is top view of a focus-sensitive pattern mask including multiplecells with differing orientations of pattern elements, in accordancewith one or more embodiments of the present disclosure.

FIG. 9B is an expanded view of a portion of a focus-sensitive patternelement including multiple segments, in accordance with one or moreembodiments of the present disclosure.

FIG. 10 is top view of a focus-sensitive printed metrology targetcorresponding to a focus-sensitive pattern mask, in accordance with oneor more embodiments of the present disclosure.

FIG. 11A is a top view of a focus-sensitive pattern mask, in accordancewith one or more embodiments of the present disclosure.

FIG. 11B is a plot illustrating pattern placement error associated withprinted pattern elements corresponding to focus-sensitive patternelements, in accordance with one or more embodiments of the presentdisclosure.

FIG. 11C is a plot illustrating pattern placement error associated withprinted pattern elements corresponding to focus-insensitive patternelements, in accordance with one or more embodiments of the presentdisclosure.

FIG. 12A is a top view of a focus-sensitive pattern element includingsub-resolution features, in accordance with one or more embodiments ofthe present disclosure.

FIG. 12B is a schematic view of a simulated printed pattern profile of aresist layer corresponding to a focus-sensitive pattern element withsub-resolution features, in accordance with one or more embodiments ofthe present disclosure.

FIG. 13 is a plot illustrating the intensity distribution of asingle-pole illumination source offset from the optical axis of the setof projection optics in both the X and Y directions, in accordance withone or more embodiments of the present disclosure.

FIG. 14 is a top view of a pattern element including multiple segmentsdistributed along both the X and Y directions, in accordance with one ormore embodiments of the present disclosure.

FIG. 15 is a plot of the distribution of diffracted beams in the pupilplane of the set of projection optics, in accordance with one or moreembodiments of the present disclosure.

FIG. 16 is a schematic view of a simulated printed pattern profile of aresist layer corresponding to a focus-sensitive pattern element withsub-resolution features, in accordance with one or more embodiments ofthe present disclosure.

FIG. 17 is a top view of a metrology target including focus-sensitiveand focus-insensitive printed pattern elements, in accordance with oneor more embodiments of the present disclosure.

FIG. 18 is a plot of an exemplary intensity distribution of anillumination source 102 for printing process-sensitive metrologytargets, in accordance with one or more embodiments of the presentdisclosure.

FIG. 19A is a conceptual view of lithography sub-system illustratingbeam paths associated with a first pole of illumination source and apattern mask configured to generate a focus-sensitive metrology targeton a sample, in accordance with one or more embodiments of the presentdisclosure.

FIG. 19B is a conceptual view of lithography sub-system illustratingbeam paths associated with a second pole of illumination sourcesymmetric to the first pole and a pattern mask configured to generate afocus-sensitive metrology target on a sample, in accordance with one ormore embodiments of the present disclosure.

FIG. 20 is a plot illustrating the distribution of diffracted beams inthe pupil plane 304 of a lithography system for the generation of afocus-sensitive metrology target, in accordance with one or moreembodiments of the present disclosure.

FIG. 21 is a schematic view of simulated printed pattern profiles of afocus exposure matrix corresponding to a focus-sensitive metrologytarget, in accordance with one or more embodiments of the presentdisclosure.

FIG. 22A is a plot illustrating the variation of the critical dimensionof printed pattern elements as a function of exposure for multiplevalues of focal position of the sample, in accordance with one or moreembodiments of the present disclosure.

FIG. 22B is a plot illustrating the variation of the sidewall angles ofprinted pattern elements as a function of exposure for multiple valuesof focal position of the sample, in accordance with one or moreembodiments of the present disclosure.

FIG. 23A is a top view of asymmetric segmented pattern elements for thegeneration of focus-sensitive metrology targets, in accordance with oneor more embodiments of the present disclosure.

FIG. 23B is a schematic view of simulated printed pattern profiles of aresist layer corresponding to a asymmetric segmented pattern elements,in accordance with one or more embodiments of the present disclosure.

FIG. 23C is a schematic view of simulated printed pattern profiles of afocus exposure matrix corresponding to images of asymmetric segmentedmetrology targets, in accordance with one or more embodiments of thepresent disclosure.

FIG. 24A is a conceptual view of lithography sub-system illustratingbeam paths associated with a first pole of illumination source and apattern mask configured to generate an exposure-sensitive metrologytarget on a sample, in accordance with one or more embodiments of thepresent disclosure.

FIG. 24B is a conceptual view of lithography sub-system illustratingbeam paths associated with a second pole of illumination sourcesymmetric to the first pole and a pattern mask configured to generate anexposure-sensitive metrology target on a sample, in accordance with oneor more embodiments of the present disclosure.

FIG. 25 is a plot illustrating the distribution of diffracted beams inthe pupil plane of a lithography system for the generation of anexposure-sensitive metrology target, in accordance with one or moreembodiments of the present disclosure.

FIG. 26 is a schematic view of simulated printed pattern of a focusexposure matrix corresponding to images of exposure-sensitive patternelements, in accordance with one or more embodiments of the presentdisclosure.

FIG. 27 is a plot illustrating a variation of the critical dimension ofexposure-sensitive printed pattern profiles as a function of exposurefor multiple values of focal position of the sample, in accordance withone or more embodiments of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings.

Embodiments of the present disclosure are directed to theco-optimization of a pattern mask and an illumination source of alithography tool to generate process-sensitive metrology targets on asample. Some embodiments of the present disclosure are directed tofocus-sensitive metrology targets in which a deviation of the focalposition of the sample within the lithography tool from a nominal focalposition is manifested as a variation of one or more characteristics ofthe focus-sensitive metrology targets that are measurable by a metrologytool. Additional embodiments of the present disclosure are directed toexposure-sensitive metrology targets in which a deviation of theexposure dose of the sample by the illumination source from a nominalvalue is manifested as a variation of one or more characteristics of theexposure-sensitive metrology targets that are measurable by a metrologytool. Some embodiments of the present disclosure are directed toasymmetric off-axis illumination sources to generate process-sensitivemetrology targets. Additional embodiments are directed to symmetricoff-axis illumination sources to generate process-sensitive metrologytargets. Further embodiments of the present disclosure are directed topattern masks with pattern elements designed based on a knownillumination profile of an illumination source to provideprocess-sensitive metrology targets.

It is recognized herein that, in the context of lithographic printing,the process window associated with the fabrication of printed featureson a sample typically defines ranges of process parameters suitable forfabrication of the printed features within a specified tolerance. Forexample, a process window may define limits on the defocus associatedwith the position of the sample along the optical axis of thelithography tool (e.g. the focal position of the sample). By way ofanother example, a process window may define limits on the dose ofenergy from the illumination source incident on the sample (e.g. theexposure of the sample). Further, the impacts of variations of multipleprocess parameters on one or more characteristics of the printedfeatures may be interdependent. In this regard, a process window mayinclude a multi-dimensional analysis of multiple process parameters(e.g. a focus-exposure matrix (FEM), or the like) to define acceptableranges of process parameters of interest. Accordingly, precisemonitoring of process parameters such as, but not limited to, focalposition of the sample and the dose of energy incident on the samplefrom an illumination source, may facilitate performance of lithographytools according to desired specifications.

It is further recognized that the degree to which characteristics of aprinted feature are robust to deviations of process parameters maydepend on a variety of factors. For example, robustness to deviations ofprocess parameters may be influenced by characteristics of the desiredpattern features such as, but not limited to, the dimensions and/or thedensity of the desired printed features. Additionally, robustness todeviations of process parameters may be influenced by opticalcharacteristics of the lithography tool such as, but not limited to, thedepth of focus (DOF), the numerical aperture (NA) of projection optics,the shape of the illumination source, the symmetry of the illuminationsource, the spectral content of the illumination source, or coherence ofthe illumination source. Further, robustness to deviations of processparameters may be influenced by characteristics of the pattern maskimaged onto the sample to generate the printed patterns such as, but notlimited to, the transmission of pattern elements, the optical phaseinduced by the pattern elements, or the dimensions of pattern elementswith respect to the resolution of the projection optics. Further, manysuch characteristics associated with the robustness of printedparameters may be interdependent.

Embodiments of the present disclosure are directed to co-optimization ofpattern elements on a pattern mask and the illumination source such thatone or more characteristics of printed patterns of metrology targets onthe sample (e.g. positions of one or more printed elements, separationdistances between printed elements, sidewall angles of printed elements,or the like) are highly sensitive to changes in process parameters (e.g.focal position of the sample, dose of illumination energy incident onthe sample, or the like). In some embodiments, pattern elements on apattern mask associated with a process-sensitive metrology target aredesigned to utilize the same illumination conditions (shape of theillumination source, symmetry of the illumination source, or the like)as printed patterns associated with fabricated devices. In this regard,a process-sensitive metrology target may be printed on a sample in thesame process step or series of process steps as pattern elementsassociated with fabricated devices. In some embodiments, patternelements on a pattern mask associated with a process-sensitive metrologytarget are designed to be utilized with a custom illumination sourceprofile. In this regard, a process-sensitive metrology target may beprinted in a dedicated process step.

Process-sensitive lithographic features are generally described in U.S.Pat. No. 6,673,638, issued on Jan. 6, 2004, which is incorporated byreference in the entirety. Focus masking structures are generallydescribed in U.S. Pat. No. 6,884,552, issued on Apr. 26, 2005, which isincorporated by reference in the entirety. Determining lithographicfocus and exposure is generally described in U.S. Pat. No. 7,382,447,issued on Jun. 3, 2008, which is incorporated by reference in theentirety. Process optimization and control using scatterometry signalsis generally described in U.S. Pat. No. 7,352,453, issued on Apr. 1,2008, which is incorporated by reference in the entirety. Detectingoverlay errors using scatterometry is generally described in U.S. Pat.No. 7,564,557, issued on Jul. 21, 2009, which is incorporated byreference in the entirety.

As used throughout the present disclosure, the term “sample” generallyrefers to a substrate formed of a semiconductor or non-semiconductormaterial (e.g. a wafer, or the like). For example, a semiconductor ornon-semiconductor material may include, but is not limited to,monocrystalline silicon, gallium arsenide, and indium phosphide. Asample may include one or more layers. For example, such layers mayinclude, but are not limited to, a resist, a dielectric material, aconductive material, and a semiconductive material. Many different typesof such layers are known in the art, and the term sample as used hereinis intended to encompass a sample on which all types of such layers maybe formed. One or more layers formed on a sample may be patterned orunpatterned. For example, a sample may include a plurality of dies, eachhaving repeatable patterned features. Formation and processing of suchlayers of material may ultimately result in completed devices. Manydifferent types of devices may be formed on a sample, and the termsample as used herein is intended to encompass a sample on which anytype of device known in the art is being fabricated. Further, for thepurposes of the present disclosure, the term sample and wafer should beinterpreted as interchangeable. In addition, for the purposes of thepresent disclosure, the terms patterning device, mask and reticle shouldbe interpreted as interchangeable.

FIG. 1A is a conceptual view illustrating a system 100 including alithography sub-system 101 for lithographically printing one or morepatterns to a sample, in accordance with one or more embodiments of thepresent disclosure. In one embodiment, the system 100 consists of alithographic sub-system 101. The lithographic sub-system 101 may includeany lithographic printing tool known in the art. For example, thelithographic sub-system 101 may include, but is not limited to, ascanner or stepper.

In another embodiment, the lithographic sub-system 101 may include anillumination source 102 configured to generate one or illumination beams104. The one or more illumination beams 104 may include one or moreselected wavelengths of light including, but not limited to, ultraviolet(UV) radiation, visible radiation, or infrared (IR) radiation. Inanother embodiment, the wavelengths of radiation of the one or moreillumination beams 104 emitted by the illumination source 102 aretunable. In this regard, the wavelengths of radiation of the one or moreillumination beams 104 may be adjusted to any selected wavelength ofradiation (e.g. UV radiation, visible radiation, infrared radiation, orthe like). In another embodiment, the illumination source 102 maygenerate one or more illumination beams 104 having any pattern known inthe art. For example, the illumination source 102 may include, but isnot limited to, a single-pole illumination source, a dipole illuminationsource, a C-Quad illumination source, a Quasar illumination source, or afree-form illumination source.

In another embodiment, the lithography sub-system 101 includes a masksupport device 106. The mask support device 106 is configured to securea pattern mask 108. In this regard, the support device 106 may hold thepattern mask 108 utilizing any means known in the art, such as, but notlimited to, a mechanical, vacuum, electrostatic or other clampingtechnique. In another embodiment, the lithography sub-system 101includes a set of projection optics 110 configured to project an imageof the pattern mask 108 illuminated by the one or more illuminationbeams 104 onto the surface of a sample 112 disposed on a sample stage114. For example, the set of projection optics 110 may be configured toproject an image of the pattern mask 108 onto a resist layer 116 on thesample 112 to generate (e.g. expose, or the like) a printed patternelement (e.g. a metrology pattern) on the resist layer 116 correspondingto a pattern element on the pattern mask 108. In another embodiment, thesupport device 106 may be configured to actuate or position the patternmask 108. For example, the support device 106 may actuate the patternmask 108 to a selected position with respect to the projection optics110 of the system 100.

The pattern mask 108 may be a reflective or a transmissive element. Inone embodiment, the pattern mask 108 is a transmissive element in whichpattern elements fully or partially block the transmission of anillumination beam 104 (e.g. through absorption or reflection of theillumination beam 104). Accordingly, the illumination beam 104 may betransmitted through spaces between pattern elements to the set ofprojection optics 110. For example, a pattern mask 108 in which patternelements fully block the transmission of the illumination beam 104 mayoperate as a binary pattern mask. It is further recognized thatfocus-sensitive binary pattern masks in which light from an illuminationsource is either fully blocked or fully transmitted/reflected togenerate an image may be advantageously utilized to determine a focalposition of a sample in a lithography system. For example, binarypattern masks are relatively inexpensive to fabricate and may be readilyincorporated into many lithography systems.

In another embodiment, features of the pattern mask 108 (e.g. patternelements, spaces between pattern elements, or the like) are designed tomodify the optical phase of an illumination beam 104. In this regard,the pattern mask 108 may operate as a phase mask (e.g. an alternatingphase shift mask, or the like).

In another embodiment, the pattern mask 108 is a reflective mask inwhich segments 202 fully or partially reflect an illumination beam 104to the set of projection optics 110 and the spaces between segments 202absorb or transmit the illumination beam 104. Further, pattern elementsof the pattern mask 108 may be formed from any opaque or semi-opaquematerial known in the art for reflecting and/or absorbing anillumination beam 104. In another embodiment, the segments 202 mayinclude a metal. For example, the segments 202 may be, but are notrequired to be, formed from chrome (e.g. a chrome alloy, or the like).

The pattern mask 108 may be utilized (e.g. by lithography sub-system101) in any imaging configuration known in the art. For example, thepattern mask 108 may be a positive mask in which pattern elements arepositively imaged as printed pattern elements of a resist layer 116 ofsample 112. By way of another example, the pattern mask 108 may be anegative mask in which pattern elements of the pattern mask 108 formnegative printed pattern elements (e.g. gaps, spaces, or the like) of aresist layer 116 of sample 112.

In another embodiment, the lithography sub-system 101 includes acontroller 118 to control the various sub-systems of the lithographysub-system 101. In another embodiment, the controller 118 includes oneor more processors 119 configured to execute program instructionsmaintained on a memory medium 120. In this regard, the one or moreprocessors 119 of controller 118 may execute any of the various processsteps described throughout the present disclosure. Further, thecontroller 118 may be communicatively coupled to the mask support device106 and/or the sample stage 114 to direct the transfer of patternelements on a pattern mask 108 to a sample 112 (e.g. a resist layer 116on the sample, or the like). It is noted herein that the lithographysub-system 101 of the present invention may implement any of the patternmask designs described throughout the present disclosure. Lee et al.generally describe mask-based lithography in U.S. Pat. No. 7,545,520,issued on Jun. 9, 2009, which is incorporated herein in the entirety.

FIG. 1B is a conceptual view illustrating a metrology sub-system 151, inaccordance with one or more embodiments of the present disclosure. Themetrology sub-system 151 may measure any metrology metric (e.g. overlayerror, CD, or the like) using any method known in the art. In oneembodiment, the metrology sub-system 151 includes an image-basedmetrology tool to measure metrology data based on the generation of oneor more images of the sample 112. In another embodiment, the metrologysub-system 151 includes a scatterometry-based metrology system tomeasure metrology data based on the scattering (reflection, diffraction,diffuse scattering, or the like) of light from the sample.

In another embodiment, the illumination source 102 directs the one ormore illumination beams 104 to the sample 112 via an illuminationpathway 121. The illumination pathway 121 may include one or more lenses122. Further, the illumination pathway 121 may include one or moreadditional optical components 124 suitable for modifying and/orconditioning the one or more illumination beams 104. For example, theone or more optical components 124 may include, but are not limited to,one or more polarizers, one or more filters, one or more beam splitters,one or more diffusers, one or more homogenizers, one or more apodizers,or one or more beam shapers. In one embodiment, the illumination pathway121 includes a beamsplitter 126. In another embodiment, the metrologysub-system 151 includes an objective lens 128 to focus the one or moreillumination beams 104 onto the sample 112.

The illumination source 102 may direct the one or more illuminationbeams 104 to the sample at any angle via the illumination pathway 121.In one embodiment, the illumination source 102 directs the one or moreillumination beams 104 to the sample 112 at normal incidence angle. Inanother embodiment, the illumination source 102 directs the one or moreillumination beams 104 to the sample 112 at an angle (e.g. a glancingangle, a 45-degree angle, or the like).

In another embodiment, the metrology sub-system 151 includes one or moredetectors 130 configured to capture radiation emanating from the sample112 through a collection pathway 132. The collection pathway 132 mayinclude multiple optical elements to direct and/or modify illuminationcollected by the objective lens 128 including, but not limited to one ormore lenses 134, one or more filters, one or more polarizers, one ormore beam blocks, or one or more beamsplitters.

For example, a detector 130 may receive an image of the sample 112provided by elements in the collection pathway 132 (e.g. the objectivelens 128, the one or more optical elements, 134, or the like). By way ofanother example, a detector 130 may receive radiation reflected orscattered (e.g. via specular reflection, diffuse reflection, and thelike) from the sample 112. By way of another example, a detector 130 mayreceive radiation generated by the sample (e.g. luminescence associatedwith absorption of the one or more illumination beams 104, and thelike). By way of another example, a detector 130 may receive one or morediffracted orders of radiation from the sample 112 (e.g. 0-orderdiffraction, ±1 order diffraction, ±2 order diffraction, and the like).Further, it is noted herein that the one or more detectors 130 mayinclude any optical detector known in the art suitable for measuringillumination received from the sample 112. For example, a detector 130may include, but is not limited to, a CCD detector, a TDI detector, aphotomultiplier tube (PMT), an avalanche photodiode (APD), or the like.In another embodiment, a detector 130 may include a spectroscopicdetector suitable for identifying wavelengths of radiation emanatingfrom the sample 112. Further, the metrology sub-system 151 may includemultiple detectors 130 (e.g. associated with multiple beam pathsgenerated by one or more beamsplitters to facilitate multiple metrologymeasurements (e.g. multiple metrology tools) by the metrology sub-system151.

In another embodiment, the metrology sub-system 151 is communicativelycoupled to the controller 118 of system 100. In this regard, thecontroller 118 may be configured to receive data including, but notlimited to, metrology data (e.g. metrology measurement results, imagesof the target, pupil images, and the like) or metrology metrics (e.g.precision, tool-induced shift, sensitivity, diffraction efficiency,through-focus slope, side wall angle, critical dimensions, and thelike).

FIG. 1C is a conceptual view illustrating a metrology sub-system 151, inaccordance with another embodiment of the present disclosure. In oneembodiment, the illumination pathway 121 and the collection pathway 132contain separate elements. For example, the illumination pathway 121 mayutilize a first focusing element 162 to focus the one or moreillumination beams 104 onto the sample 112 and the collection pathway132 may utilize a second focusing element 164 to collect radiation fromthe sample 112. In this regard, the numerical apertures of the firstfocusing element 162 and the second focusing element 164 may bedifferent. Further, it is noted herein that the metrology sub-system 151depicted in FIG. 1C may facilitate multi-angle illumination of thesample 106, and/or more than one illumination source 102 (e.g. coupledto one or more additional detectors 120). In this regard, the metrologysub-system 151 depicted in FIG. 1B may perform multiple metrologymeasurements. In another embodiment, one or more optical components maybe mounted to a rotatable arm (not shown) pivoting around the sample 112such that the angle of incidence of the one or more illumination beams104 on the sample 112 may be controlled by the position of the rotatablearm.

FIG. 2 is a schematic view illustrating a pattern mask including asegmented pattern element, in accordance with one or more embodiments ofthe present disclosure. In one embodiment, the segmented pattern element200 of pattern mask 108 includes a plurality of periodically distributedsegments 202 having a pitch 204 (e.g. line/space (LS) pattern elements).In this regard, segmented pattern element 200 may induce diffraction ofone or more illumination beams 104. For example, an illumination beam104 incident on the pattern mask 200 may be diffracted into multiplediffracted beams separated along the X-direction corresponding tomultiple diffraction orders (e.g. 0 order, ±1 order, ±2 order, and thelike). In another embodiment, a width of segments 206 along theX-direction is equal to a separation distance 208 between segments suchthat pattern element 200 is a 1:1 line/space target. In anotherembodiment, the width of segments 206 along the X-direction is not equalto a separation distance 208 between segments.

FIGS. 3A through 3C illustrate the co-optimization of the illuminationsource 102 and the pattern mask 108 to generate an asymmetric aerialimaging profile on a sample 112 for a focus-sensitive metrology target,in accordance with one or more embodiments of the present disclosure.

FIG. 3A is a conceptual view of the lithography sub-system 101illustrating multiple diffracted beams generated by a pattern mask 108,in accordance with one or more embodiments of the present disclosure. Inone embodiment, the illumination source 102 generates an off-axisillumination beam 104. In another embodiment, the pattern mask 108diffracts the incident illumination beam 104 to generate multiplediffracted beams including, but not limited to, a 0-order diffractedbeam 306, a 1^(st) order diffracted beam 308 and a 2^(nd) orderdiffracted beam 310. In another embodiment, two of the diffracted beams(e.g. 306 and 308) are captured by the set of projection optics 110 anddirected to the sample 112 (e.g. a resist layer 116 of sample 112) togenerate an aerial image of the pattern mask 108 on the sample 112.

FIG. 3B is a conceptual view of a pupil plane of the set of projectionoptics 110 illustrating relative positions of diffracted beams withinthe pupil plane, in accordance with one or more embodiments of thepresent disclosure. In one embodiment, the pupil plane 304 ischaracterized as a circle with radius 1 (e.g. a pupil limit 312) suchthat illumination from the sample within the pupil limit 312 is capturedby the set of projection optics 110 and illumination falling outside thepupil limit 312 is not captured by the set of projection optics 110 andthus does not contribute to the generation of the aerial image of thepattern mask 108 on the sample 112.

For example, the 0-order diffracted beam 306 and the 1^(st) orderdiffracted beam 308 may lie within the pupil limit 312 of pupil plane304 (e.g. an entrance pupil plane as illustrated in FIG. 3A, an exitpupil plane, or the like) and are thus captured by the set of projectionoptics 110, while the 2^(nd) order diffracted beam 310 is outside thepupil limit 312 and is not captured by the set of projection optics.Further, a location of a portion of a diffracted beam (e.g. any ofdiffracted beams 306-310) within the pupil plane 304 may be described asa radial position, σ, with respect to the center of the pupil plane 304(e.g. optical axis 136). For example, the width of diffracted beam 306along the X-direction may be described as a difference between an outerposition, σ_(out), and an inner position, σ_(in), of the diffracted beam306.

In another embodiment, the distribution of diffracted beams 306,308 inthe pupil plane 304 provides asymmetric illumination of the sample 112.For example, as illustrated in FIG. 3B, the diffracted beams 306-310 maybe distributed such that the 0-order diffracted beam 306 is located onan internal edge of the pupil limit 312, whereas the 2^(nd) orderdiffracted beam 312 lies on the outer edge of the pupil limit 312 and isnot captured by the set of projection optics. Accordingly, the set ofprojection optics 110 may generate an aerial image using the highlyoff-axis 0-order diffracted beam 306 and a slightly off-axis 1^(st)order diffracted beam 308 such that the optical paths of the diffractedbeams 306,308 are asymmetrically incident on the sample 112 (e.g. asshown in FIG. 1).

In another embodiment, an asymmetric distribution of diffracted beamswithin the pupil plane 310 (e.g. within the pupil limits 312) providesasymmetric illumination of the sample 112 associated with the generationof the aerial image of the pattern mask 108. FIG. 3C is a plotillustrating the relative acid concentration 314 within a resist layer116 of sample 112 exposed by asymmetric illumination as shown in FIGS.3A and 3B, in accordance with one or more embodiments of the presentdisclosure. In this regard, FIG. 3C may be a latent image representativeof the spatial distribution of the aerial image within the resist layer116. It is noted herein that the asymmetric illumination may generate anasymmetric exposure profile of the resist layer 116. Accordinglydevelopment of the resist layer 116 may generate asymmetric printedelements having one or more characteristics (e.g. a position of the topof the corresponding printed element, a critical dimension associatedwith the separation of printed elements, one or more sidewall angles, orthe like) that vary as a function of the focal position of the sample112 along the optic axis 136. In this regard, the corresponding printedpattern elements may operate as focus-sensitive printed patternelements.

It is noted herein that the separation of diffracted beams, the numberof diffracted beams captured by the projection optics 110, and therelative positions of the captured diffracted beams within the pupilplane 304 may be controlled by adjusting parameters associated with theillumination source 102 and the pattern mask 108 (e.g. by co-optimizingthe illumination source and the pattern mask 108). In this regard, theseparation of diffracted beams, the number of diffracted beams capturedby the projection optics 110, and the relative positions of the captureddiffracted beams within the pupil plane 304 may be determined at leastin part by the illumination source 102 and the pattern mask 108. Forexample, a 0-order diffracted beam 306 may propagate along a straightpath from the illumination source 102 through the pattern mask 108 (e.g.undiffracted) to the set of projection optics 110. Accordingly, a shapeof an illumination beam 104 (e.g. the diameter of an illumination pole,or the like) as well as an off-axis pole distance 314 (e.g. a distancebetween the illumination pole 104 and the optic axis 136 of the set ofprojection optics 110) may determine the position of the 0-orderdiffracted beam 306 in the pupil plane 304. By way of another example,the positions of higher-order diffracted beams (e.g. 1^(st) orderdiffracted beam 308, 2^(nd) order diffracted beam 310, or the like) inthe pupil plane 304 are determined by a pitch of pattern elements of thepattern mask 108 (e.g. pitch 204 of segmented pattern element 200, orthe like) as well as the off-axis pole distance 302.

FIGS. 3D through 3F illustrate the co-optimization of the illuminationsource 102 and the pattern mask 108 to generate a second asymmetricaerial imaging profile on a sample 112 for a focus-sensitive metrologytarget, in accordance with one or more embodiments of the presentdisclosure.

FIG. 3D is a conceptual view of the lithography sub-system 101illustrating multiple diffracted beams generated by a pattern mask 108,in accordance with one or more embodiments of the present disclosure.FIG. 3E is a conceptual view of a pupil plane of the set of projectionoptics 110 illustrating the relative positions of diffracted beamswithin the pupil plane, in accordance with one or more embodiments ofthe present disclosure. FIG. 3F is a plot illustrating the relative acidconcentration 316 within a resist layer 116 of sample 112 exposed byasymmetric illumination as shown in FIGS. 3D and 3E, in accordance withone or more embodiments of the present disclosure.

The degree of asymmetry of the illumination of the sample 112illustrated by FIGS. 3D through 3F may be reduced relative to theconfiguration illustrated in FIGS. 3A through 3D. For example, as shownin FIGS. 3D and 3E, the distribution of diffracted beams 306,308 may beless asymmetric with respect to the optic axis 136. The reducedasymmetry of FIG. 3E relative to 3B may result in a reduced asymmetry ofthe relative acid concentration within a resist layer 116 of sample 112as illustrated in FIG. 3F. In this regard, the degree of asymmetry ofdiffracted beams present in the pupil plane 304 may correlate to thedegree of asymmetric illumination of the sample. Accordingly, thesensitivity of one or more characteristics of printed pattern elementsto the focal position of the sample 112 may be adjusted by controllingthe distribution of diffracted beams in the pupil plane 304. It is notedthat the degree of asymmetric of printed pattern elements may negativelyimpact the robustness of the printed pattern elements. For example,highly asymmetric printed pattern elements may be prone to collapse. Inthis regard, the distribution of diffracted beams captured by the set ofprojection optics may be adjusted (e.g. by co-optimization of theillumination source 102 and the pattern mask 108) to provide a balancebetween focus-sensitivity and robustness of printed pattern elements.

FIGS. 3G through 3I illustrate the co-optimization of the illuminationsource 102 and the pattern mask 108 to generate a symmetric aerialimaging profile on a sample 112 for a focus-insensitive metrologytarget, in accordance with one or more embodiments of the presentdisclosure.

FIG. 3G is a conceptual view of the lithography sub-system 101illustrating multiple diffracted beams generated by a pattern mask 108,in accordance with one or more embodiments of the present disclosure.FIG. 3H is a conceptual view of a pupil plane of the set of projectionoptics 110 illustrating the relative positions of symmetric diffractedbeams within the pupil plane, in accordance with one or more embodimentsof the present disclosure. FIG. 3I is a plot illustrating the relativeacid concentration 318 within a resist layer 116 of sample 112 exposedby symmetric illumination as shown in FIGS. 3D and 3E, in accordancewith one or more embodiments of the present disclosure. In anotherembodiment, a symmetric distribution of diffracted beams 306,308 in thepupil plane 304 provides symmetric illumination of the sample 112associated with the aerial image of the pattern mask 108. Accordingly,printed pattern elements associated with the symmetric illumination willoperate as focus-insensitive printed pattern elements. In anotherembodiment, focus-insensitive printed pattern elements may be fabricatedwithin the same metrology target (e.g. in the same process step or in adifferent process step). For example, a focus-insensitive printedpattern element may provide a reference against which one or morecharacteristics of a focus-sensitive printed pattern element may bemeasured. By way of another example, a focus-insensitive printed patternelement may facilitate the determination of an overlay measurement (e.g.a translation of one patterned layer on the sample 116 with respect toone or more additional pattern layers) in addition to a measurement ofthe focal position of the sample 112.

FIGS. 4A through 6 illustrate the variation of printed pattern elementsfabricated using asymmetric illumination as a function of the pitch ofthe pattern mask, in accordance with one or more embodiments of thepresent disclosure. In this regard, FIGS. 4A through 6 illustrate one ormore embodiments of the present disclosure in which the characteristicsof the illumination source 102 are fixed at known values. Accordingly,the pitch of one or more pattern elements on pattern mask 108 may beadjusted to control the asymmetry of illumination on the sample 112 andthus the sensitivity of the corresponding printed pattern elements todeviations of the focal position of the sample 112. It is noted hereinthat FIGS. 4A through 6 and the associated description below areprovided solely for illustrative purposes and should not be interpretedas limiting.

FIGS. 4A through 4D are plots 402-408 illustrating the distribution ofdiffracted beams in a pupil plane 304 associated with an off-axissingle-pole illumination source and a pattern mask 108 with values ofpitch of 80 nm, 100 nm, 140 nm, and 150 nm, in accordance with one ormore embodiments of the present disclosure. For example, FIGS. 4Athrough 4D may illustrate diffracted beams captured by the set ofprojection optics 110. FIG. 5 is a schematic view of simulated printedpattern profiles 500 (e.g. in a resist layer 116 of sample 112) forpattern masks 108 having pitch values ranging from 80 nm to 160 nm, inaccordance with one or more embodiments of the present disclosure. Forexample, simulated printed pattern profiles associated with pitch valuesof 80 nm, 100 nm, 140 nm, and 150 nm may correspond to distributions ofdiffracted beams illustrated in FIGS. 4A through 4D, respectively. Inanother embodiment, box 502 may illustrate a process window depictingasymmetric pattern resist profiles for use as focus-sensitive patternelements. FIG. 6 is a plot 600 illustrating pattern placement error(PPE) associated with a measurement of the deviation of the top of theprinted patterns for pitch values ranging from 75 nm to 140 nm, inaccordance with one or more embodiments of the present disclosure. Forexample, PPE may be measured as a deviation of the top of printedpattern elements in response to a deviation of the focal position of thesample.

In one embodiment, as illustrated in FIG. 4A, a pattern element pitch of80 nm provides a symmetric distribution of a 0-order diffracted beam 410and a 1^(st) order diffracted beam 412 in the pupil plane 304.Accordingly, as illustrated in FIG. 5, the resulting printed patternprofiles are symmetric. Further, as illustrated in FIG. 6, the resultingprinted pattern profile do not exhibit any PPE as a function of focalposition of the sample (e.g. defocus) and may operate as focusinsensitive pattern elements.

In another embodiment, as illustrated in FIGS. 4B and 4C, increasing thepitch from 80 nm decreases the spacing between diffracted orders. Forexample, increasing the pitch causes the 1^(st) diffracted beam to movein a direction towards the 0-order diffracted beam 410 in the pupilplane 304. As previously described, the position of the 0-orderdiffracted beam within the pupil plane 304 may, but is not required to,be unaffected by the pitch of the pattern mask 108. Further, as thedistribution of diffracted beams in the pupil plane 304 becomesincreasingly asymmetric (e.g. as illustrated in FIGS. 4B and 4C), thecorresponding printed pattern profiles may become increasingly symmetric(e.g. see printed pattern profiles corresponding to 90 nm through 140 nmin FIG. 5) as evidenced by asymmetric sidewall angles, a deviation ofthe top of the printed pattern profile relative to the bottom of theprinted pattern profile, or the like. Further, the sensitivity of thePPE vs defocus (e.g. the slope associated with plots in FIG. 6) mayincrease with increasing asymmetry of printed pattern profiles.

In another embodiment, as illustrated in FIG. 4D, increasing the pitchto 150 nm and beyond may decrease the spacing of diffracted beams410-414 in the pupil plane 304 such that 2^(nd) order diffracted beam414 is captured by the set of projection optics 110. As a result, theasymmetry of the illumination of the sample 112 may be reduced and theprinted pattern profiles for pitch values of 150 nm and 160 nm shown inFIG. 5 may be correspondingly reduced.

In another embodiment, a pattern mask 108 includes a characteristicdesign of an overlay metrology target such that a deviation of the focalposition of the sample 112 is manifested as a measurable translation ofone or more printed pattern elements. For example, a typical imagingmetrology overlay target (e.g. an Advanced Imaging Metrology (AIM)target, a box-in box, target, a scatterometry overlay target, or thelike) may include one or more printed pattern elements associated withone or more processing steps such that an overlay error (e.g. atranslation of one layer with respect to another) is manifested as arelative translation between pattern elements of the imaging metrologyoverlay target. Correspondingly, a focus-sensitive metrology target maybe designed to mimic an imaging metrology overlay target such that adeviation of the focal position of the sample is manifested as ameasurable translation of one or more printed pattern elements of thefocus-sensitive metrology target. It is noted herein that afocus-sensitive mask may be designed to mimic any overlay metrologytarget including, but not limited to, imaging metrology overlay targetsor scatterometry metrology overlay targets. It is further noted that afocus-sensitive metrology target that mimics an overlay metrology targetmay be readily characterized by an overlay metrology tool (e.g. a customoverlay metrology tool, a commercially available metrology tool, or thelike). Further, the output of the overlay metrology tool may be furtheranalyzed (e.g. by controller 118) to convert a measured “overlay error”to the focal position of the sample 112 when the pattern mask 108 wasimaged (e.g. by lithography sub-system 101). For example, thefocus-sensitive metrology target may be designed such that no measuredoverlay error corresponds to a sample positioned at a nominal (ordesired) focal position. In this regard, a measured overlay error by anoverlay metrology tool may correspond to an error (e.g. an offset) inthe focal position of the sample relative to the nominal position.

In another embodiment, a focus-sensitive pattern mask may be used togenerate a corresponding printed metrology target for any number ofprocess layers on a target. For example, a focus-sensitive mask may beused to generate a metrology target suitable for characterizing thefocal position of the sample for the single layer. By way of anotherexample, a focus-sensitive mask may be used to generate a metrologytarget suitable for characterizing the focal position of the sample forany number of process layers. In this regard, a focus-sensitive patternmask may be used to generate printed pattern elements in one or morelayers a metrology target, and a focus-insensitive pattern mask (e.g. apattern mask with symmetric elements, or the like) may be used togenerate printed pattern elements in one or more additional layers ofthe metrology target. In another embodiment, a single metrology targetmay include pattern elements associated with both focus-sensitive andfocus-insensitive pattern masks. Accordingly, printed pattern elementsassociated with focus-insensitive pattern masks may serve as points ofreference for the measurement of relative position of printed patternelements associated with focus-sensitive pattern masks. Further, ametrology target including printed pattern elements associated with bothfocus-sensitive and focus-insensitive pattern masks may simultaneouslyprovide traditional overlay data (e.g. translations between one or moreprocess layers on the sample) and the focal position of the sample 112for one or more process layers.

FIGS. 7A through 7C illustrate a focus-sensitive pattern mask 700, inaccordance with one or more embodiments of the present disclosure. FIG.7A is a top view of a focus-sensitive pattern mask 700 includingfocus-sensitive pattern elements 702 and focus-insensitive patternelements 704, in accordance with one or more embodiments of the presentdisclosure. FIG. 7B is an enlarged view 706 of a portion of afocus-sensitive pattern element 702, in accordance with one or moreembodiments of the present disclosure. FIG. 7C is an enlarged view 708of a portion of a focus-insensitive pattern element 704, in accordancewith one or more embodiments of the present disclosure.

In one embodiment, as illustrated in FIG. 7B, each focus-sensitivepattern element 702 is a segmented pattern element including multiplesegments 714 distributed with a focus-sensitive pitch 712. For example,the multiple segments 714 distributed with pitch 712 may diffractillumination beam 104 and generate an asymmetric distribution ofdiffracted beams in the pupil plane 304 of lithography system 101.Accordingly, the aerial image of pattern elements 702 may be asymmetricsuch that the positions of corresponding printed pattern elements aresensitive to the focal position of the sample 112. In anotherembodiment, as illustrated in FIG. 7A, the segmented pattern elements702 are further distributed with a focus-insensitive pitch 710. Forexample, pitch 710 may be greater than pitch 712 such that diffractedbeams associated with pitch 710 are not collected by the set ofprojection optics 110 and thus do not influence the symmetry of theillumination of the sample.

In another embodiment, as illustrated in FIG. 7C, each focus-insensitivepattern element 704 is an unsegmented pattern element. Further,focus-insensitive pattern elements 704 may be distributed withfocus-insensitive pitch 710. Accordingly, aerial images of patternelements 704 generated by the set of projection optics 110 asilluminated by the illumination beam 104 provide focus-insensitiveprinted patterns on the sample 112. For example, pattern elements 704illuminated by illumination source 102 and imaged by the set ofprojection optics 110 may provide a symmetric aerial illuminationprofile on the sample 112. In another embodiment, focus-insensitivepattern elements 704 distributed with focus-insensitive pitch 710 arealigned with focus-sensitive pattern elements 702. In this regard,printed pattern elements associated with focus-sensitive patternelements 702 and focus-insensitive pattern elements 704 may be alignedwhen the sample 112 is located at a nominal focal position andmisaligned otherwise.

Focus-sensitive pattern elements 702 and focus-insensitive patternelements 704 may be imaged onto the sample 112 to generate printedpattern elements in a single processing step or in multiple processingsteps. In one embodiment, focus-sensitive pattern elements 702 andfocus-insensitive pattern elements 704 are located on a single patternmask (e.g. pattern mask 700) and simultaneously imaged onto the sample112. In another embodiment, focus-sensitive pattern elements 702 andfocus-insensitive pattern elements 704 may be separately imaged onto thesample 112. For example, focus-sensitive pattern elements 702 andfocus-insensitive pattern elements 104 may be located on separatepattern masks or different locations of a single pattern mask.

FIG. 8A is a top view of a focus-sensitive printed metrology target 800corresponding to focus-sensitive pattern mask 700, in accordance withone or more embodiments of the present disclosure. For example,focus-sensitive metrology target 800 may correspond to an aerial imageof focus-sensitive pattern mask 700 by an illumination source 102configured for single-pole illumination in which the illumination poleis offset from the optical axis 136 of lithography sub-system 101 in theX-direction. In one embodiment, focus-sensitive printed pattern elements802 correspond to focus-sensitive pattern elements 702 andfocus-insensitive printed pattern elements 804 correspond tofocus-insensitive pattern elements 704. It is noted herein that,although not shown for clarity, the multiple segments 714 offocus-sensitive pattern elements 702 may be, but are not required to be,resolved by the set of projection optics 110 and printed as separateprinted pattern elements on the sample 112. As illustrated in FIG. 8A, amisalignment metric 806 (e.g. a PPE measured in the X-direction) maycorrespond to a deviation of the focal position of the sample 112 withrespect to a nominal focal position.

FIG. 8B is a plot 808 illustrating an exemplary relationship between thefocal position of a sample 112 and a misalignment metric 806, inaccordance with one or more embodiments of the present disclosure. Asshown in FIG. 8B, a misalignment metric 806 may, but is not required to,exhibit a linear dependence with respect to the focal position of thesample 112. In one embodiment, a focal position of 0 represents anominal focal position (e.g. a target focal position, a “best” focalposition, or the like).

In another embodiment, a misalignment metric 806 is measured bymetrology sub-system 151. For example, metrology sub-system 151 may beconfigured as an overlay metrology tool to measure the misalignmentmetric 806. Further, the actual focal position of the sample 112 (e.g. amagnitude and/or a direction of a deviation of the focal position of thesample 112) may be calculated (e.g. by controller 118) based on themeasured misalignment metric. It is noted herein that the description ofmisalignment metric 806 is provided solely for illustrative purposes andshould not be interpreted as limiting. For example, any metrology metricmay be utilized to characterize the focal position of the sample 112. Inone embodiment, translations of the locations of individual segments ofprinted pattern elements 802 (not shown) may be utilized to characterizethe focal position of the sample 112. In another embodiment, one or moreadditional characteristics of the printed pattern elements 802 such as,but not limited to, one or more side-wall angles or one or more criticaldimensions may be utilized to characterize the focal position of thesample 112.

FIG. 9A is top view of a focus-sensitive pattern mask 900 includingmultiple cells 902-908 with differing orientations of pattern elements,in accordance with one or more embodiments of the present disclosure. Inone embodiment, focus-sensitive pattern mask 900 may be characteristicof a two-layer AIM metrology overlay target. For example, pattern mask900 may include focus-sensitive pattern elements and focus-insensitivepattern elements in each of cells 902-908.

In another embodiment, cell 902 corresponds to focus-sensitive patternmask 700. In this regard, focus-sensitive pattern elements 910 may besegmented pattern elements including a focus-sensitive pitch. Further,both focus-sensitive pattern elements 910 and focus-insensitive patternelements 912 may be aligned on the pattern mask 900. Additionally, cell906 may be an additional instance of cell 902.

In another embodiment, as illustrated in expanded view 918 in FIG. 9B,each focus-sensitive pattern element 914 is a segmented pattern elementincluding multiple segments 920 distributed with a focus-sensitive pitch922. For example, the multiple segments 920 distributed withfocus-sensitive pitch 922 may diffract illumination beam 104 andgenerate an asymmetric distribution of diffracted beams in the pupilplane 304 of lithography system 101. For example, the multiple segments920 may be distributed along the X-direction to correspond to the offsetposition 302 of illumination source 102. In this regard, the patternmask 900 and the illumination source 102 may be co-optimized.Accordingly, the aerial image of pattern elements 914 may be asymmetricsuch that the positions of corresponding printed pattern elements aresensitive to the focal position of the sample 112.

FIG. 10 is top view of a focus-sensitive printed metrology target 1000corresponding to cell 904 of focus-sensitive pattern mask 900, inaccordance with one or more embodiments of the present disclosure. Inone embodiment, focus-sensitive printed pattern elements 1002 correspondto focus-sensitive pattern elements 910 and focus-insensitive printedpattern elements 1004 correspond to focus-insensitive pattern elements916. For example, printed In another embodiment, misalignment metric1006 (e.g. a relative separation between focus-sensitive printed patternelements 1002 and focus-insensitive printed pattern elements 1004, orthe like) provides data on the deviation of the focal position of thesample 112.

FIG. 11A is a top view of a focus-sensitive pattern mask 1100, inaccordance with one or more embodiments of the present disclosure. Inone embodiment, pattern elements in region 1102 are distributed with afocus-sensitive pitch 1104. In another embodiment, pattern elements inregion 1106 are distributed with a focus-insensitive pitch 1108. In thisregard, one or more characteristics of pattern elements corresponding toregion 1102 (e.g. PPE associated with a location of correspondingprinted pattern elements, a critical dimension, a sidewall angle, or thelike) may vary with respect to the focal position of the sample 112.Further, the characteristics of printed pattern elements correspondingto region 1106 may be constant with respect to the focal position of thesample 112.

FIGS. 11B and 11C are plots 1110 and 1112 illustrating the PPEassociated with printed pattern elements corresponding tofocus-sensitive pattern elements in region 1102 and focus-insensitivepattern elements in region 1104, respectively, in accordance with one ormore embodiments of the present disclosure. For example, the PPE offocus-sensitive printed pattern elements corresponding to region 1102may, but are not required to, exhibit a linear (or nearly linear)dependence on the focal position of the sample 112 over a range ofinterest. Further, the PPE of focus-insensitive printed pattern elementscorresponding to region 1106 may exhibit a negligible PPE with respectto the focal position of the sample 112. It is noted herein that thescale of plot 1112 is magnified with respect to the scale of plot 1110.Further, the PPE of focus-sensitive printed pattern elementscorresponding to region 1102 may be measured with respect tofocus-insensitive printed pattern elements corresponding to region 1106.

FIG. 12A is a top view of a focus-sensitive pattern element 1200including sub-resolution features, in accordance with one or moreembodiments of the present disclosure. In one embodiment, patternelement 1200 is a segmented pattern element including multiple segments1202 distributed (e.g. along the X-direction) with a sub-resolutionseparation distance 1204. In this regard, sub-resolution features (e.g.separation distance 1204, or the like) may not be printed on the sample112 by the lithography sub-system 101. In another embodiment, thesegments 1202 are distributed (e.g. along the X-direction) with afocus-sensitive pitch 1206. Accordingly, pattern element 1200 maydiffract illumination beam 104 such that multiple diffracted beams areasymmetrically distributed in the pupil plane 304 of the lithographysub-system. Further, the illumination on the sample 112 associated withthe aerial image of pattern element 1200 may be asymmetric such that theprinted pattern element is sensitive to the focal position of the sample112. It is noted herein that a pattern element including sub-resolutionfeatures that induce diffraction of illumination beam but are not imagedonto the sample may facilitate the fabrication of robust printed patternelements. For example, highly asymmetric printed pattern elements mayexhibit highly asymmetric sidewall angles. Accordingly, highlyasymmetric printed pattern elements may be prone to collapse,particularly when the aspect ratio (e.g. a ratio of a printed patternheight to a printed pattern width) is high. In contrast, the multiplesegments (e.g. segments 1202) of pattern elements includingsub-resolution features may be imaged as a single printed patternelement with a relatively low aspect ratio to facilitate robustness ofthe printed pattern element, while simultaneously providing sensitivityto the focal position of the sample 112.

FIG. 12B is a schematic view of a simulated printed pattern profile 1208of a resist layer 116 corresponding to a focus-sensitive pattern element1200 with sub-resolution features, in accordance with one or moreembodiments of the present disclosure. In another embodiment, theprinted pattern corresponding to the segmented focus-sensitive patternelement 1200 is an unsegmented printed pattern element. In this regard,the sub-resolution features (e.g. sub-resolution separation distances1204, or the like) may not be resolvably imaged by the set of projectionoptics 110 of the lithography sub-system 101. However, as illustrated inFIG. 12B, the printed pattern profile 1208 of the resist layer 116 maybe asymmetric due to the influence of the sub-resolution features. Inone embodiment, a PPE (e.g. as measured by a location of the top of theprinted pattern element) is sensitive to the focal position of thesample 112. In another embodiment, one or more sidewall angles may besensitive to the focal position of the sample 112.

FIGS. 13 through 17 illustrate an illumination source and a pattern maskco-optimized to generate printed pattern elements sensitive to the focalposition of the sample as measured along two directions, in accordancewith one or more embodiments of the present disclosure.

FIG. 13 is a plot 1300 illustrating the intensity distribution of anillumination source 102 configured as a single-pole illumination sourceoffset from the optical axis 136 of the set of projection optics 110 inboth the X and Y directions, in accordance with one or more embodimentsof the present disclosure.

FIG. 14 is a top view of a pattern element 1400 including multiplesegments 1402 distributed along both the X and Y directions, inaccordance with one or more embodiments of the present disclosure. Forexample, pattern element 1400 may include a two-dimensional array ofsegments 1402 separated by focus-sensitive pitch 1404 in the X-directionand focus-sensitive pitch 1406 in the Y-direction to generate multiplediffracted beams distributed in both the X and Y directions. In oneembodiment, the values of the focus-sensitive pitch in the X-direction1404 and in the Y-direction 1406 are the same to provide the same degreeof asymmetry of illumination on the sample 112 along the X and Ydirections. In another embodiment, the values of the focus-sensitivepitch in the X-direction 1404 and in the Y-direction 1406 are thedifferent to provide the different amounts degree of asymmetry ofillumination on the sample 112 along the X and Y directions. In anotherembodiment, the intensity profile of the illumination source 102 mayinclude differing degrees of asymmetry in the X and Y directions, whichmay be compensated for by differing values of the focus-sensitive pitchin the X-direction 1404 and the Y-direction 1406 to provide the samedegree of asymmetry of illumination on the sample 112 along the X and Ydirections.

In one embodiment, the segments 1402 are separated by a separationdistance 1408 along the X-direction and a separation distance 1410 alongthe Y-direction. In another embodiment, the separation distances1408,1410 are larger than the resolution of the set of projection optics110 such that each segment 1402 is resolvably imaged onto the sample 112as a printed pattern element. In another embodiment, the separationdistances 1408,1410 are smaller than the resolution of the set ofprojection optics 110 such that the multiple segment 1402 are imagedonto the sample 112 as a single printed pattern element.

FIG. 15 is a plot 1500 of the distribution of diffracted beams in thepupil plane 304 of the set of projection optics 110, in accordance withone or more embodiments of the present disclosure. In one embodiment,pattern element 1400 diffracts illumination beam 104 into a 0-orderdiffracted beam 1502, a 1^(st) order diffracted beam 1504 in theY-direction and a 1^(st) order diffracted beam 1506 in the X-directionsuch that the diffracted beams are asymmetrically distributed in thepupil plane 304. In another embodiment, the pupil plane 304 includes oneor more additional diffracted beams (e.g. diffracted beam 1508).

FIG. 16 is a schematic view of a simulated printed pattern profile 1600of a resist layer 116 corresponding to a focus-sensitive pattern element1400 with sub-resolution features, in accordance with one or moreembodiments of the present disclosure. In one embodiment, asymmetricillumination associated with an asymmetric distribution of diffractedbeams 1502-1508 induces a resist profile 1600 that is asymmetric in boththe X and Y directions. For example, resist profile 1600 may exhibitasymmetric sidewall angles in both the X and Y directions in responds todeviations of the focal position of the sample 112, leading to PPE.Further, the top of the resist profile 1600 may shift in both the X andY directions in responds to deviations of the focal position of thesample 112, leading to PPE.

FIG. 17 is a top view of a metrology target 1700 includingfocus-sensitive and focus-insensitive printed pattern elements, inaccordance with one or more embodiments of the present disclosure. Inone embodiment, metrology target 1700 includes focus-sensitive printedpattern element 1702 corresponding to an image of pattern element 1400.For example, printed pattern element 1702 may be configured to providesensitivity to the focal position of the sample 112 as measured alongboth the X and Y directions. In another embodiment, metrology target1700 includes a focus-insensitive printed pattern element 1704surrounding the focus-sensitive printed pattern element 1702. In thisregard, pattern mask 1700 may be characteristic of a box-in-box overlaymetrology target. For example, a deviation of the focal position of thesample 112 along the X-direction may be manifested as a shift of theprinted pattern element 1702 and thus a change in distance 1706.Similarly, a deviation of the focal position of the sample 112 along theY-direction may be manifested as a shift of the printed pattern element1702 and thus a change in distance 1708. In this regard, metrologytarget 1700 is sensitive to the pitch and yaw of the sample 112 as wellas an average focal position of the sample 112.

Focus-sensitive pattern element 1702 and focus-insensitive patternelement 1704 may be imaged onto the sample 112 to generate printedpattern elements in a single processing step or in multiple processingsteps. In one embodiment, focus-sensitive pattern element 1702 andfocus-insensitive pattern elements 1704 are located on a single patternmask and simultaneously imaged onto the sample 112. In anotherembodiment, focus-sensitive pattern element 1702 and focus-insensitivepattern element 1704 may be separately imaged onto the sample 112. Forexample, focus-sensitive pattern element 1702 and focus-insensitivepattern element 104 may be located on separate pattern masks ordifferent locations of a single pattern mask.

In some embodiments, the illumination source 102 is configured toexhibit a symmetric off-axis intensity distribution. In this regard, thedistribution of the illumination source 102 may be suitable for thefabrication of printed pattern elements associated with semiconductordevices of interest as well as process-sensitive metrology targets. Forexample, a symmetric off-axis illumination source 102 such as, but notlimited to, a symmetric dipole illumination source 102 may be suitablefor printing dense line/space patterns (e.g. associated with fins, gatesor the like of logic and/or memory devices). Accordingly, in someembodiments, the pattern mask 108 and the symmetric illumination source102 are co-optimized to provide process-sensitive metrology targetssuitable for fabrication on a sample 112 in the same set of processsteps used to fabricate semiconductor devices.

FIG. 18 is a plot 1800 of an exemplary intensity distribution of anillumination source 102 for printing process-sensitive metrologytargets, in accordance with one or more embodiments of the presentdisclosure. In one embodiment, illumination source 102 is a symmetricdipole illumination source including a first pole 104 a and a secondpole 104 b symmetric to the first pole. It is noted, however, that theexemplary intensity distribution of FIG. 18 is provided solely forillustrative purposes and should not be interpreted as limiting. Forexample, the illumination source 102 may be configured as any symmetricsource including, but not limited to, a symmetric dipole source, aC-Quad illumination source, a Quasar illumination source, or a free-formillumination source with a symmetric distribution.

It is noted herein that each illumination pole (e.g. illumination poles104 a,104 b) may be diffracted by the pattern mask 108 according to thedistribution of pattern elements on the pattern mask 108. In thisregard, diffracted beams associated with a given illumination pole mayinterfere on the sample 112 such that the intensity distribution on thesample 112 associated with the given illumination pole is a function ofthe relative optical phase (e.g. associated with optical pathdifferences, or the like) of the diffracted beams. Further, the totalintensity distribution of may include contributions of the first andsecond illumination poles (e.g. illumination poles 104 a,104 b).

For example, the intensity of the illumination on the sample 112associated with a first illumination pole (e.g. illumination pole 104 a)may be described as the interference of two diffracted beams at thesample 112 (e.g. in a lithography system 101 as illustrated in FIG. 3A,or the like):

l ₁(x,z)=a ₀ ² +a ₁ ²+2a ₀ a ₁ cos(ΔΦ(x,z))  (1)

where a₀ and a₁ are constants associated with the electric fieldamplitude of first and second diffracted beams, and ΔΦ corresponds to anoptical phase difference between the diffracted beams. For example, thediffracted beams may include a 0-order diffracted beam and a 1^(st)order diffracted beam (e.g. as shown in FIGS. 3A,3D,3G, or the like). Inthis regard, ΔΦ(x,z) may be described as:

$\begin{matrix}{{{\Delta\Phi}\left( {x,z} \right)} = {{{\frac{2\pi}{p}x} + {\frac{2\pi \; n}{\lambda}\left( {{\cos \; \theta_{1}} - {\cos \; \theta_{0}}} \right)z}} = {{\frac{2\pi}{p}x} + {\frac{2\pi}{\lambda}\left( {\sqrt{1 - \left( {{\sin \; \theta_{0}} - \frac{\lambda}{p}} \right)^{2}} - {\cos \; \theta_{0}}} \right)z}}}} & (2)\end{matrix}$

where p is a pitch of the pattern mask 108 along the x-direction, λ isthe wavelength of the illumination beam 104, θ₀ is the diffraction angleof the 0-order diffracted beam from the pattern mask 108, and θ₁ is thediffraction angle of the 1^(st) order diffracted beam from the patternmask 108. Further, the diffraction angle of the 0-order diffracted beam,θ₀, may be the same as the incident angle of the illumination beam 104.Accordingly, the 0-order diffracted beam may propagate along a linearpath through the pattern mask 108.

The total intensity distribution on the sample 112 associated withsymmetric dipole illumination may thus be described by the contributionsof the intensity distributions from the two dipole sources. For example,the total intensity distribution may be, but is not required to be,described as:

$\begin{matrix}{{I_{Tot}\left( {x,z} \right)} = {\frac{{I_{1}(x)} + {I_{2}(x)}}{2} = {a_{0}^{2} + a_{1}^{2} + {2a_{0}a_{1}{\cos \left( {\frac{2\pi}{p}x} \right)}{{\cos \left( {\frac{2\pi}{\lambda}\left( {\sqrt{1 - \left( {{\sin \; \theta_{0}} - \frac{\lambda}{p}} \right)^{2}} - {\cos \; \theta_{0}}} \right)z} \right)}.}}}}} & (3)\end{matrix}$

In this regard, the intensity distribution on the sample 112 maycorrespond to a sinusoidal distribution along the X-direction with aperiod equal to the pitch, p, of the pattern mask 108. Further, theintensity distribution on the sample 112 along the Z-direction maycorrespond to a sinusoidal distribution with a period a function of theincident angle of the illumination beam 104, θ₀, and the pitch of thepattern mask 108, p.

It is to be understood that equations 1 through 3 and the associateddescription of the distribution of illumination on the sample 112associated with the illumination source 104 is provided solely forillustrative purposes and should not be interpreted as limiting. Forexample, the illumination source 102 may exhibit any spatial and/ortemporal coherence properties to provide a desired illumination profileon the sample 112. In this regard, the spatial and/or temporal coherenceproperties of the illumination source 102 may affect the interferencebetween diffracted orders of a given illumination pole and/or theinterference between multiple illumination poles. Further, the totalintensity distribution on the sample 112 may include contributions fromany number of diffracted beams from any number of locations on theillumination source (e.g. illumination poles, or the like).

In one embodiment, the illumination source 102 and the pattern mask 108may be co-optimized to provide an intensity distribution on the sample112 suitable for generating a focus-sensitive metrology target. Forexample, the illumination source 102 and the pattern mask 108 may beco-optimized to generate an optical phase difference between diffractedbeams from each of a pair of symmetric illumination poles (e.g. suchthat ΔΦ≠0 in equations 1-3). Accordingly, as shown by equations 1-3, theintensity on the sample 112 associated with the first dipole (e.g.l₁(x,z)) may be asymmetric in a first direction, whereas the intensityon the sample 112 associated with the second dipole (e.g. l₂(x,z)) maybe asymmetric in the opposite direction. Further, the total intensitydistribution on the sample 112 (e.g. l_(Tot)(x,z)) may be modulatedalong the Z-direction (e.g. along the optical axis of the set ofprojection optics 110, or the like). In this regard, the intensitydistribution on the sample 112 may be sensitive to the focal position ofthe sample 112 (e.g. according to a Z-dependent term in equation 3, orthe like). Accordingly, deviations of the focal position of the samplemay influence one or more characteristics of printed pattern elements onthe sample 112 to generate a focus-sensitive metrology target.

FIG. 19A is a conceptual view of lithography sub-system 101 illustratingbeam paths associated with a first pole of illumination source 102 and apattern mask 108 configured to generate a focus-sensitive metrologytarget on a sample 112, in accordance with one or more embodiments ofthe present disclosure. FIG. 19B is a conceptual view of lithographysub-system 101 illustrating beam paths associated with a second pole ofillumination source 102 symmetric to the first pole and a pattern mask108 configured to generate a focus-sensitive metrology target on asample 112, in accordance with one or more embodiments of the presentdisclosure. It is noted herein that beam paths associated with bothFIGS. 19A and 19B, as well as additional pairs of symmetric illuminationpoles (not shown) may be simultaneously present to generate an aerialimage of pattern mask 108 on the sample 112.

FIG. 20 is a plot 2000 illustrating the distribution of diffracted beams306 a,306 b,308 a,308 b in the pupil plane 304 of a lithography system101 for the generation of a focus-sensitive metrology target, inaccordance with one or more embodiments of the present disclosure. Inone embodiment, diffracted beams 306 a,306 b correspond to beam pathsillustrated in FIG. 19A. For example, diffracted beam 306 a maycorrespond to a 0-order diffracted beam and diffracted beam 306 b maycorrespond to a 1^(st) order diffracted beam. Further, diffracted beams306 a,306 b may be distributed with an asymmetric profile such that theoptical paths of the diffracted beams through the lithography sub-system101 differ (e.g. θ₀≠θ₁ in equation 2) and the illumination of the sample112 may be asymmetric. Similarly, diffracted beams 308 a,308 b maycorrespond to beam paths illustrated in FIG. 19B. For example,diffracted beam 308 a may correspond to a 0-order diffracted beam anddiffracted beam 308 b may correspond to a 1^(st) order diffracted beam.Further, diffracted beams 308 a,308 b may be similarly distributed withan asymmetric profile such that the illumination of the sample 112 maybe asymmetric. However, the combined distribution of diffracted beams306 a,306 b,308 a,308 b may be symmetric.

In another embodiment, diffracted beams 306 a,306 b,308 a,308 b may havea finite width in the pupil plane 304 corresponding to a spatial extentof the illumination poles of the illumination source 102. For example, adistribution of diffracted beams a pitch of a pattern mask 108 suitablefor generating a focus-sensitive metrology target on the sample 112 maybe calculated based on a known distribution of the illumination source102. For example, the pattern mask 108 may be, but is not required tobe, designed such that a focus-sensitive pupil separation distance,D_(f), between diffracted beams of a given illumination pole (e.g. theseparation between 306 a and 306 b) in the pupil plane 304 may becalculated as:

$\begin{matrix}{D_{f} = {\frac{\sigma_{out} + \sigma_{in}}{2} + \left( {\sigma_{out} - \sigma_{in}} \right)}} & (4)\end{matrix}$

where σ_(out) and σ_(in) are the outer and inner extents of the 0-orderdiffracted beam in the pupil plane, respectively. Accordingly, thediffracted beams of each illumination pole may be asymmetricallydistributed in the pupil plane without overlap (e.g. as illustrated inFIG. 20).

Further, a focus-sensitive pitch, P_(f), of pattern elements on thepattern mask 108 may be calculated based on the focus-sensitive pupilseparation distance, D_(f). In one embodiment, the separation betweendiffracted beams generated by a pattern mask 108 may be describedaccording to a diffraction equation:

$\begin{matrix}{\frac{m\; \lambda}{P} = {{{n\; \sin \; \theta_{0}} + {n\; \sin \; \theta_{1}}} = {{NA}\left( {\sigma_{0} + \sigma_{1}} \right)}}} & (5)\end{matrix}$

where λ is the wavelength of the illumination source 102, n is therefractive index surrounding the diffraction grating, p is a pitchassociated with pattern elements on the pattern mask 108, θ₀ is theincident angle 1902 of the illumination beam 104 as well as the exitangle of the 0-order diffracted beam (e.g. diffracted beam 306 a,308 a),θ₁ is the exit angle 1904 of a diffracted beam (e.g. diffracted beam 306b,308 b), σ₀ is a center position 1906 of the 0-order diffracted beam(e.g. diffracted beam 306 a,308 a) in the pupil plane 304, σ₁ is acenter position 1908 of the 1^(st) order diffracted beam (e.g.diffracted beam 306 a,308 a) in the pupil plane 304, and NA is thenumerical aperture of the set of projection optics 110. In anotherembodiment, a focus-sensitive pitch, P_(f), may be calculated based onthe focus-sensitive pupil separation, D_(f), between a 0-orderdiffracted beam (e.g. diffracted beam 306 a,308 a) and a 1^(st) orderdiffracted beam (e.g. diffracted beam 306 a,308 a where m=1) as:

$\begin{matrix}{P_{f} = {\frac{\lambda}{{NA} \cdot D_{f}}.}} & (6)\end{matrix}$

By way of an illustrative example, lithography sub-system 101 mayinclude a symmetric dipole source (e.g. corresponding to plot 1800 ofFIG. 18, or the like) configured for the fabrication of line/spacepatterns with a pitch of 80 nm. Further, the 0-order diffracted beams306 a,308 a in the pupil plane may have an outer extent, σ_(out), of0.96 and an inner extent, σ_(in), of 0.86 such that the 0-orderdiffracted beams 306 a,308 a are distributed near the pupil limit 312.Accordingly, the pattern mask 108 may be designed to generate diffractedorders with a separation of D_(f)=0.942 (e.g. according to equation 4)with a pitch, Pf=152 nm (e.g. according to equation 6).

FIG. 21 is a schematic view of simulated printed pattern profiles 2100(e.g. in a resist layer 116 of sample 112) of a focus exposure matrix(FEM) corresponding to a focus-sensitive metrology target, in accordancewith one or more embodiments of the present disclosure. In oneembodiment, FIG. 21 illustrates variations of printed pattern profileswith respect to the focal position of the sample along the horizontalaxis and the exposure of the sample (e.g. the dose of energy incident onthe sample by the illumination beams 104) along the vertical axis. Inanother embodiment, a process window of interest is illustrated by thebox 2102. For example, printed pattern profiles 2100 may correspond toprinted patterns fabricated with a focus-sensitive pitch of 150 nm asdescribed above. Further, multiple characteristics of the printedpattern elements such as, but not limited to the printed height,sidewall angle, and critical dimension (e.g. a width of the printedpattern at a designated height), are sensitive to deviations of thefocal position of the sample.

In another embodiment, focus-sensitive characteristics (e.g. height,sidewall angle, critical dimension, or the like) are relativelysensitive to deviations of the focal position of the sample, andrelatively insensitive to other process variations such as, but notlimited to the exposure of the sample. For example, as illustrated inFIG. 21, within the process window 2102, the printed pattern profiles2100 exhibit a higher sensitivity to deviations of the focal position ofthe sample 112 (e.g. as viewed along the horizontal axis) than theexposure (e.g. as viewed along the vertical axis).

FIG. 22A is a plot 2200 illustrating the variation of a criticaldimension of printed pattern elements (e.g. printed pattern elements2100) as a function of exposure for multiple values of focal position ofthe sample 112, in accordance with one or more embodiments of thepresent disclosure. For example, plot 2200 may correspond to measuredvalues of the simulated printed pattern profiles of FIG. 21. In oneembodiment, the sensitivity of the critical dimension to focal positionof the sample 112 varies by an exemplary 18 nm in response to a 70 nmdeviation of the focal position. Further, for a given focal position ofthe sample 112, the variations of the critical dimension as a functionof exposure are relatively small (e.g. approximately 5 nanometers overthe range of interest).

FIG. 22B is a plot 2202 illustrating the variation of the sidewallangles of printed pattern elements (e.g. printed pattern elements 2100)as a function of exposure for multiple values of focal position of thesample 112, in accordance with one or more embodiments of the presentdisclosure. For example, plot 2202 may correspond to measured values ofthe simulated printed pattern profiles of FIG. 21. In one embodiment,the sidewall angle varies by an exemplary 16 degrees in response to a 70nm deviation of the focal position of the sample 112. Further, for agiven focal position of the sample 112, the variations of the sidewallangle as a function of exposure are relatively small (e.g. approximately3 degrees over the range of interest).

In another embodiment, multiple characteristics of printed patternelements may be simultaneously utilized to determine deviations of thefocal position of the sample 112. For example, measurements of thecritical dimension and the sidewall angle of printed pattern elements ofa metrology target may provide greater sensitivity and accuracy thanmeasurements of a single characteristic.

It is noted herein that the image of a pattern mask generated on asample (e.g. by lithography sub-system 101) may critically depend on theproximity of pattern elements within a pattern mask. In this regard,pattern elements with dimensions (e.g. actual dimensions, separationsbetween pattern elements, or the like) smaller than a resolution of thelithography sub-system 101 (e.g. the set of projection optics 110) mayinfluence a pattern printed on a resist layer of a sample based onoptical effects such as scattering, diffraction, and the like. Further,sub-resolution pattern elements (alternatively, optical proximitycorrection (OPC) pattern elements, or the like) may influence one ormore characteristics of printed pattern elements (e.g. PPE, sidewallangle, critical dimension, or the like) without being resolvably imagedonto the sample.

FIG. 23A is a top view of asymmetric segmented pattern elements 2302 forthe generation of focus-sensitive metrology targets, in accordance withone or more embodiments of the present disclosure. In one embodiment,asymmetric segmented pattern elements 2302 are separated by afocus-sensitive pitch 2310 such that diffracted beams from anillumination pole (e.g. an off-axis single-pole illumination source asillustrated by FIG. 3A, one of a pair of symmetric illumination poles asillustrated in FIGS. 19A and 19B, or the like) are asymmetricallydistributed in the pupil plane 304 of lithography sub-system 101. Inanother embodiment, asymmetric segmented pattern elements 2302 includeone or more features (e.g. segments, separation distances betweensegments, or the like) smaller than the resolution of the set ofprojection optics 110. For example, as shown in FIG. 23A, asymmetricsegmented pattern element 2302 includes a primary segment 2304 and asecondary segment 2306 separated along the X-direction by asub-resolution separation distance 2308. Further, the lengths of theprimary segment 2304 and the secondary segment 2306 along theX-direction may be different such that pattern element 2302 isasymmetric in the X-direction. Accordingly, the asymmetric segmentedpattern elements 2302 may be imaged onto the sample 112 as unsegmentedprinted pattern elements.

In another embodiment, asymmetric segmented pattern elements 2302separated by a focus-sensitive pitch 2310 and illuminated by a symmetricillumination source 102 (e.g. as illustrated in FIGS. 19A and 19B)provide asymmetric illumination of the sample 112 associated with theaerial image of the pattern mask 108. For example, asymmetric segmentedpattern elements 2302 may break the symmetry of illumination beam 104 aand illumination beam 104 b to provide asymmetric illumination of thesample 112. Accordingly, corresponding printed pattern elements may beasymmetric.

FIG. 23B is a schematic view of simulated printed pattern profiles 2312of a resist layer 116 corresponding to a asymmetric segmented patternelements 2302, in accordance with one or more embodiments of the presentdisclosure. In one embodiment, the sidewall angles as well as the shapesof the printed pattern profiles 2312 are asymmetric.

FIG. 23C is a schematic view of simulated printed pattern profiles 2314(e.g. in a resist layer 116 of sample 112) of a focus exposure matrixcorresponding to images of asymmetric segmented metrology targets, inaccordance with one or more embodiments of the present disclosure. Forexample, simulated printed pattern profiles 2314 may correspond toimages of asymmetric segmented pattern elements 2302. In one embodiment,FIG. 23C illustrates variations of printed pattern profiles with respectto the focal position of the sample 112 along the horizontal axis andthe exposure of the sample 112 (e.g. the dose of energy incident on thesample 112 by the illumination beams 104) along the vertical axis. Inanother embodiment, a process window 2316 illustrates process parametersof interest. For example, the process window 2316 may include robustprinted pattern elements and/or practical ranges associated withexpected deviations of the focal position of the sample 112 and theexposure of the sample.

In another embodiment, the asymmetric printed pattern profiles 2314 arehighly sensitive to deviations of the focal position of the sample andare insensitive to deviations of the exposure. In this regard, theasymmetric printed pattern profiles 2314 may operate as focus-sensitivepatterns on a focus-sensitive metrology target.

In one embodiment, the illumination source 102 and the pattern mask 108may be co-optimized to provide an intensity distribution on the sample112 suitable for generating an exposure-sensitive metrology target.

FIG. 24A is a conceptual view of lithography sub-system 101 illustratingbeam paths associated with a first pole of illumination source 102 and apattern mask 108 configured to generate an exposure-sensitive metrologytarget on a sample 112, in accordance with one or more embodiments ofthe present disclosure. FIG. 24B is a conceptual view of lithographysub-system 101 illustrating beam paths associated with a second pole ofillumination source 102 symmetric to the first pole and a pattern mask108 configured to generate an exposure-sensitive metrology target on asample 112, in accordance with one or more embodiments of the presentdisclosure. It is noted herein that beam paths associated with bothFIGS. 24A and 24B, as well as additional pairs of symmetric illuminationpoles (not shown) may be simultaneously present to generate an aerialimage of pattern mask 108 on the sample 112.

In one embodiment, the illumination source 102 (e.g. the symmetricillumination source illustrated in FIGS. 24A and 24B, or the like) andthe pattern mask 108 are co-optimized such that diffracted beams fromeach of a pair of symmetric illumination poles have the same opticalpath length when propagating through the lithography sub-system 101. Forexample, the illumination source 102 and the pattern mask 108 may beco-optimized such that diffracted beams are symmetrically distributed inthe pupil plane 304 (e.g. θ₀=θ₁ according to equation 2). Accordingly,as shown by equation 2, the optical phase between the diffracted beams,ΔΦ, may not exhibit a dependence on the Z-direction. The total intensitydistribution on the sample may thus be, but is not required to be,described as:

$\begin{matrix}{{I_{Tot}\left( {x,z} \right)} = {a_{0}^{2} + a_{1}^{2} + {2a_{0}a_{1}{{\cos \left( {\frac{2\pi}{p}x} \right)}.}}}} & (7)\end{matrix}$

In this regard, the total intensity distribution on the sample 112 maybe insensitive to deviations of the focal position of the sample (e.g.along the Z-direction) such that any variations of characteristics ofprinted pattern elements may be attributed to deviations of the exposureof the sample 112.

FIG. 25 is a plot 2500 illustrating the distribution of diffracted beams306 a,306 b,308 a,308 b in the pupil plane 304 of a lithography system101 for the generation of an exposure-sensitive metrology target, inaccordance with one or more embodiments of the present disclosure. Inone embodiment, diffracted beams 306 a,306 b correspond to beam pathsillustrated in FIG. 24A. For example, diffracted beam 306 a maycorrespond to a 0-order diffracted beam and diffracted beam 306 b maycorrespond to a 1^(st) order diffracted beam. Further, diffracted beams306 a,306 b may be symmetrically distributed such that the optical phasedifference between the diffracted beams 306 a,306 b is zero (e.g. θ₀=θ₁)and the illumination of the sample is symmetric. Similarly, diffractedbeams 308 a,308 b may correspond to beam paths illustrated in FIG. 24B.For example, diffracted beam 308 a may correspond to a 0-orderdiffracted beam and diffracted beam 308 b may correspond to a 1^(st)order diffracted beam. Further, diffracted beams 308 a,308 b may besymmetrically distributed such that the optical path difference betweenthe diffracted beams 308 a,308 b is zero (e.g. θ₀=θ₁) and theillumination of the sample is symmetric. Additionally, the beams fromthe two illumination poles may overlap. For example, as illustrated inFIG. 25, the diffracted beam 306 a and diffracted beam 308 b mayoverlap. Similarly, diffracted beam 306 b and 308 a may overlap.

In another embodiment, the separation of diffracted beams in the pupilplane 304 is designed to achieve a relatively high depth of field on thesample 102. For example, the separation of diffracted beams in the pupilplane 304 may be configured to be equal to the separation of theillumination poles of the illumination source 102. In this regard, thesensitivity of printed pattern elements to deviations of the focalposition of the sample 112 may be reduced. Accordingly, one or morecharacteristics of printed pattern elements may be sensitive to theexposure (e.g. dose) on the sample.

In another embodiment, an exposure-sensitive pupil separation distance,D_(e), between a 0-order diffracted beam (e.g. diffracted beam 306 a,308a) and a 1^(st) order diffracted beam (e.g. diffracted beam 306 a,308 a)may be calculated as:

D _(e)=2σ₀=2σ₁=2σ  (8)

where σ₀ is a center position 2402 of the 0-order diffracted beam (e.g.diffracted beam 306 a,308 a) in the pupil plane 304, and σ₁ is a centerposition 2402 of the 1^(st) order diffracted beam (e.g. diffracted beam306 a,308 a) in the pupil plane 304. In another embodiment, anexposure-sensitive pitch, P_(e), of pattern elements on the pattern mask108 may be calculated as (e.g. according to equation 5):

$\begin{matrix}{P_{e} = {\frac{\lambda}{2{{NA} \cdot \sigma}}.}} & (9)\end{matrix}$

Further, the width of the diffracted beams in the pupil plane may definea process window for the design of exposure-sensitive metrology targets.For example, a process window may include values of exposure-sensitivepitch, P_(e), ranging from:

$\begin{matrix}{P_{e,\min} = \frac{\lambda}{2{{NA} \cdot \sigma_{in}}}} & (10) \\{P_{e,\max} = \frac{\lambda}{2{{NA} \cdot \sigma_{out}}}} & (11)\end{matrix}$

where σ_(in) and σ_(out) are the inner and outer extents of the 0-orderdiffracted beam in the pupil plane 304, respectively.

By way of an illustrative example, lithography sub-system 101 mayinclude a symmetric dipole source (e.g. corresponding to plot 1800 ofFIG. 18, or the like) configured for the fabrication of line/spacepatterns with a pitch of 80 nm. Further, the 0-order diffracted beams306 a,308 a in the pupil plane may have an outer extent, σ_(out), of0.96 and an inner extent, σ_(in), of 0.86 such that the 0-orderdiffracted beams 306 a,308 a are distributed near the pupil limit 312.Accordingly, the pattern mask 108 may be designed to have anexposure-sensitive pitch, P_(e), in the range of 74-84 nm. In thisregard, an exposure-sensitive pitch of 80 nm may correspond to the samefeature size as the line/space patterns to be fabricated as part of asemiconductor device on the sample 112.

FIG. 26 is a schematic view of simulated printed pattern profiles 2314(e.g. in a resist layer 116 of sample 112) of a focus exposure matrixcorresponding to images of exposure-sensitive pattern elements, inaccordance with one or more embodiments of the present disclosure. Forexample, simulated printed pattern profiles 2602 may correspond toillumination of the sample 112 with a distribution of diffracted beams306 a,306 b,308 a,308 b as depicted in FIG. 25, in accordance with oneor more embodiments of the present disclosure. In one embodiment, FIG.26 illustrates variations of printed pattern profiles with respect tothe focal position of the sample 112 along the horizontal axis and theexposure of the sample 112 (e.g. the dose of energy incident on thesample 112 by the illumination beams 104) along the vertical axis. Inanother embodiment, a process window 2604 illustrates process parametersof interest. For example, the process window 2604 may include robustprinted pattern elements and/or practical ranges associated withexpected deviations of the focal position of the sample 112 and theexposure of the sample.

In another embodiment, the printed pattern profiles 2602 are highlysensitive to deviations of the exposure position of the sample and areinsensitive to deviations of the focal position of the sample 112.

FIG. 27 is a plot 2700 illustrating a variation of a critical dimensionof exposure-sensitive printed pattern profiles (e.g. a width of printedpattern profiles as measured at a designated height) as a function ofexposure for multiple values of focal position of the sample 112, inaccordance with one or more embodiments of the present disclosure. Inone embodiment, the critical dimension is sensitive to the exposure andexhibits a relatively low sensitive to the focal position of the sample.For example, in a process window 2702, the critical dimension varies by3.3 nm in response to a deviation of the exposure of 2 mJ/cm². Incontrast, the critical dimension varies by 0.001 nm in response to a 70nm deviation of the focal position of the sample 112. In this regard,the printed pattern profiles 2314 exhibit high sensitivity to exposureand minimal sensitivity to deviations of the focal position of thesample 112 and may thus operate as exposure-sensitive patterns on anexposure-sensitive metrology target.

Referring again to FIGS. 1A through 1C, the illumination source 102 mayinclude any illumination source known in the art suitable for generatingan illumination beam 104. For example, the illumination source 102 mayinclude, but is not limited to, a monochromatic light source (e.g. alaser), a polychromatic light source with a spectrum including two ormore discrete wavelengths, a broadband light source, or awavelength-sweeping light source. Further, the illumination source 102may be, but is not required to be, formed from a white light source(e.g. a broadband light source with a spectrum including visiblewavelengths), a laser source, a free-form illumination source, asingle-pole illumination source, a multi-pole illumination source, anarc lamp, an electrode-less lamp, or a laser sustained plasma (LSP)source. Further, the illumination beam 104 may be delivered viafree-space propagation or guided light (e.g. an optical fiber, a lightpipe, or the like).

It is further noted herein that, for the purposes of the presentdisclosure, an illumination pole of the illumination source 102 mayrepresent illumination from a specific location of the illuminationsource 102. In this regard, each spatial location on an illuminationsource 102 may be considered an illumination pole. Further, anillumination pole may have any shape or size known in the art.Additionally, a free-form illumination source 102 may be considered tohave an illumination profile corresponding to a distribution ofillumination poles.

In another embodiment, the system 100 includes a sample stage 114suitable for securing a sample 112. The sample stage 114 may include anysample stage architecture known in the art. For example, the samplestage 114 may include, but is not limited to, a linear stage. By way ofanother example, the stage assembly 118 may include, but is not limitedto, a rotational stage. Further, the sample 106 may include a wafer,such as, but not limited to, a semiconductor wafer.

In another embodiment, the angle of incidence of the illumination beam104 on the sample 112 is adjustable. For example, the path of theillumination beam 104 through the beamsplitter 126 and the objectivelens 128 may be adjusted to control the angle of incidence of theillumination beam 104 on the sample 112. In this regard, theillumination beam 104 may have a nominal path through the beamsplitter126 and the objective lens 128 such that the illumination beam 104 has anormal incidence angle on the sample 112. Further, the angle ofincidence of the illumination beam 104 on the sample 112 may becontrolled by modifying the position and/or angle of the illuminationbeam 104 on the beamsplitter 126 (e.g. by rotatable mirrors, a spatiallight modulator, a free-form illumination source, or the like).

The one or more processors 119 of a controller 118 may include anyprocessing element known in the art. In this sense, the one or moreprocessors 119 may include any microprocessor-type device configured toexecute algorithms and/or instructions. In one embodiment, the one ormore processors 119 may consist of a desktop computer, mainframecomputer system, workstation, image computer, parallel processor, or anyother computer system (e.g., networked computer) configured to execute aprogram configured to operate the system 100, as described throughoutthe present disclosure. It is further recognized that the term“processor” may be broadly defined to encompass any device having one ormore processing elements, which execute program instructions from anon-transitory memory medium 120. Further, the steps describedthroughout the present disclosure may be carried out by a singlecontroller 118 or, alternatively, multiple controllers 118.Additionally, the controller 118 may include one or more controllers 118housed in a common housing or within multiple housings. In this way, anycontroller or combination of controllers may be separately packaged as amodule suitable for integration into system 100.

The memory medium 120 may include any storage medium known in the artsuitable for storing program instructions executable by the associatedone or more processors 119. For example, the memory medium 120 mayinclude a non-transitory memory medium. By way of another example, thememory medium 134 may include, but is not limited to, a read-onlymemory, a random access memory, a magnetic or optical memory device(e.g., disk), a magnetic tape, a solid state drive and the like. It isfurther noted that memory medium 120 may be housed in a commoncontroller housing with the one or more processors 119. In oneembodiment, the memory medium 120 may be located remotely with respectto the physical location of the one or more processors 119 andcontroller 118. For instance, the one or more processors 119 ofcontroller 118 may access a remote memory (e.g., server), accessiblethrough a network (e.g., internet, intranet and the like). Therefore,the above description should not be interpreted as a limitation on thepresent invention but merely an illustration.

In another embodiment, the controller 118 directs the illuminationsource 102 to provide one or more selected wavelengths of illumination(e.g. in response to feedback). In a general sense, the controller 118may be communicatively coupled with any element within the metrologysub-system 101. In another embodiment, the controller 118 iscommunicatively coupled to the optical components 162 and/or theillumination source 102 to direct the adjustment of the angle ofincidence between the illumination beam 104 and the sample 112. Further,the controller 118 may analyze data received from the detector 130 andfeed the data to additional components within the metrology sub-system101 or external to the system 100.

Embodiments of the present disclosure may incorporate any type ofmetrology system known in the art including, but not limited to, aspectroscopic ellipsometer with one or more angles of illumination, aspectroscopic ellipsometer for measuring Mueller matrix elements (e.g.using rotating compensators), a single-wavelength ellipsometer, anangle-resolved ellipsometer (e.g. a beam-profile ellipsometer), aspectroscopic reflectometer, a single-wavelength reflectometer, anangle-resolved reflectometer (e.g. a beam-profile reflectometer), animaging system, a pupil imaging system, a spectral imaging system, or ascatterometer. Further, the metrology system may include a singlemetrology tool or multiple metrology tools. A metrology systemincorporating multiple metrology tools is generally described in U.S.Pat. No. 7,478,019. Focused beam ellipsometry based on primarilyreflective optics is generally described in U.S. Pat. No. 5,608,526,which is incorporated herein by reference in its entirety. The use ofapodizers to mitigate the effects of optical diffraction causing thespread of the illumination spot beyond the size defined by geometricoptics is generally described in U.S. Pat. No. 5,859,424, which isincorporated herein by reference in its entirety. The use ofhigh-numerical-aperture tools with simultaneous multipleangle-of-incidence illumination is generally described by U.S. Pat. No.6,429,943, which is incorporated herein by reference in its entirety.

It is further recognized herein that a metrology tool may measurecharacteristics of one or more targets such as, but not limited to,critical dimensions (CD), overlay, sidewall angles, film thicknesses, orprocess-related parameters (e.g. focus, dose, and the like). The targetsmay include certain regions of interest that are periodic in nature,such as for example gratings in a memory die. The metrology targets mayfurther possess various spatial characteristics and are typicallyconstructed of one or more cells which may include features in one ormore layers which may have been printed in one or more lithographicallydistinct exposures. The targets or the cells may possess varioussymmetries such as two-fold or four-fold rotation symmetry, reflectionsymmetry. Examples of such metrology structures are described in U.S.Pat. No. 6,985,618, which is included herein by reference in itsentirety. Different cells or combinations of cells may belong todistinct layers or exposure steps. The individual cells may compriseeither isolated non-periodic features or alternately they may beconstructed from one, two or three dimensional periodic structures orcombinations of non-periodic and periodic structures. The periodicstructures may be non-segmented or they may be constructed from finelysegmented features which may at or close to the minimum design rule ofthe lithographic process used to print them. The metrology targets mayalso be collocated or in close proximity with dummification structuresin the same layer or in a layer above, below or in between the layers ofthe metrology structures. Targets can include multiple layers (e.g.films) whose thicknesses can be measured by the metrology tool. Targetscan include target designs placed on the semiconductor wafer for use(e.g., with alignment, overlay registration operations, and the like).Further, targets may be located at multiple sites on the semiconductorwafer. For example, targets may be located within scribe lines (e.g.,between dies) and/or located in the die itself. Multiple targets may bemeasured simultaneously or serially by the same or multiple metrologytools as described in U.S. Pat. No. 7,478,019, which is incorporatedherein by reference in its entirety.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, other components. It isto be understood that such depicted architectures are merely exemplary,and that in fact many other architectures can be implemented whichachieve the same functionality. In a conceptual sense, any arrangementof components to achieve the same functionality is effectively“associated” such that the desired functionality is achieved. Hence, anytwo components herein combined to achieve a particular functionality canbe seen as “associated with” each other such that the desiredfunctionality is achieved, irrespective of architectures or intermedialcomponents. Likewise, any two components so associated can also beviewed as being “connected”, or “coupled”, to each other to achieve thedesired functionality, and any two components capable of being soassociated can also be viewed as being “couplable”, to each other toachieve the desired functionality. Specific examples of couplableinclude but are not limited to physically interactable and/or physicallyinteracting components and/or wirelessly interactable and/or wirelesslyinteracting components and/or logically interactable and/or logicallyinteracting components.

It is believed that the present disclosure and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components without departing from the disclosedsubject matter or without sacrificing all of its material advantages.The form described is merely explanatory, and it is the intention of thefollowing claims to encompass and include such changes. Furthermore, itis to be understood that the invention is defined by the appendedclaims.

What is claimed:
 1. A lithography system, comprising: an illuminationsource configured to direct a beam of illumination from an off-axisillumination pole to a pattern mask, wherein the pattern mask includes aset of pattern elements configured to generate a set of diffracted beamsincluding illumination from the illumination pole; and a set ofprojection optics, wherein at least two diffracted beams of the set ofdiffracted beams received by the set of projection optics areasymmetrically distributed in a pupil plane of the set of projectionoptics, wherein the at least two diffracted beams of the set ofdiffracted beams are asymmetrically incident on the sample to form a setof fabricated elements corresponding to an image of the set of patternelements, wherein the set of fabricated elements on the sample includesone or more indicators of a location of the sample along an optical axisof the set of projection optics.
 2. The lithography system of claim 1,wherein the pattern mask further comprises: an additional set of patternelements configured to generate an additional set of diffracted beams,wherein the additional set of diffracted beams includes illuminationfrom the illumination pole diffracted by the additional set of patternelements, wherein at least two diffracted beams of the additional set ofdiffracted beams received by the set of projection optics aresymmetrically distributed in the pupil plane, wherein the at least twodiffracted beams of the additional set of diffracted beams form anadditional set of fabricated features on the sample corresponding to animage of the additional set of pattern elements, wherein the additionalset of fabricated features are constant with respect to the location ofthe sample along the optical axis of the set of projection optics. 3.The lithography system of claim 1, wherein the off-axis illuminationsource includes an off-axis illumination pole.
 4. The lithography systemof claim 3, wherein the set of pattern elements is distributed along afirst direction, wherein the off-axis illumination pole is offset fromthe optical axis of the set of projection optics along the firstdirection.
 5. The lithography system of claim 4, wherein the off-axisillumination pole is further offset from the optical axis of the set ofprojection optics along a second direction perpendicular to the firstdirection.
 6. The lithography system of claim 5, wherein the one or moreindicators of the location of the sample include one or more indicatorsmeasurable along the first direction and one or more indicatorsmeasurable along the second direction.
 7. The lithography system ofclaim 1, wherein the off-axis illumination source is further configuredto direct an additional beam of illumination from an additional off-axisillumination pole symmetric to the off-axis illumination pole, whereinthe set of pattern elements generate an additional set of diffractedbeams including illumination from the additional illumination pole,wherein at least two additional diffracted beams of the additional setof diffracted beams received by the set of projection optics areasymmetrically distributed in the pupil plane of the set of projectionoptics, wherein the at least two additional diffracted beams contributeto the formation of the set of fabricated features on the samplecorresponding to the image of the set of pattern elements.
 8. Thelithography system of claim 7, wherein the at least two diffracted beamsof the set of diffracted beams and the at least two diffracted beams ofthe additional set of diffracted beams are distributed in anon-overlapping pattern in the pupil plane.
 9. The lithography system ofclaim 7, wherein at least one pattern element of the set of patternelements of the pattern mask further comprise: one or moreasymmetrically-distributed features smaller than a resolution of the setof projection optics, wherein a fabricated element of the set offabricated elements corresponding to an image of the at least onepattern element of the set of pattern elements is asymmetric.
 10. Thelithography system of claim 1, wherein the one or more indicators of thelocation of the sample comprise: at least one of a position of the setof fabricated elements, a distance between two elements of the set offabricated elements, or a sidewall angle of at least one of the set offabricated elements.
 11. The lithography system of claim 1, wherein theset of pattern elements comprises: a substantially opaque material. 12.The lithography system of claim 11, wherein the substantially opaquematerial includes a metal.
 13. The lithography system of claim 1,wherein the set of pattern element comprises: at least one of a binarypattern element or a phase mask pattern element.
 14. A lithographysystem, comprising: an off-axis illumination source, wherein theillumination source includes a first illumination pole and a secondillumination pole, wherein the first and second illumination poles aresymmetrically distributed with respect to an optical axis, wherein theoff-axis illumination source is configured to direct illumination fromthe first and second illumination poles to a pattern mask; wherein thepattern mask includes a set of pattern elements, wherein the set ofpattern elements is configured to generate a first set of diffractedbeams including illumination from a first illumination pole diffractedfrom the set of pattern elements, wherein the set of pattern elements isconfigured to generate a second set of diffracted beams includingillumination from the second illumination pole; and a set of projectionoptics, wherein at least two diffracted beams of the first set ofdiffracted beams received by the set of projection optics aresymmetrically distributed in a pupil plane of the set of projectionoptics, wherein at least two diffracted beams of the second set ofdiffracted beams received by the set of projection optics overlap thefirst set of diffracted beams in the pupil plane, wherein the at leasttwo diffracted beams of the first and second sets of diffracted beams ofthe set of diffracted beams form a set of fabricated elements on thesample corresponding to an image of the set of pattern elements, whereinthe set of fabricated elements on the sample includes one or moreindicators of a dose of illumination on the sample associated with atleast two diffracted beams of the first and second sets of diffractedbeams.
 15. The lithography system of claim 14, wherein the at least twodiffracted beams of the first set of diffracted beams and the at leasttwo diffracted beams of the second set of diffracted beams aredistributed in a non-overlapping pattern in the pupil plane.
 16. Thelithography system of claim 14, wherein at least one pattern element ofthe set of pattern elements of the pattern mask further comprise: one ormore asymmetrically-distributed features smaller than a resolution ofthe set of projection optics, wherein a fabricated element of the set offabricated elements corresponding to an image of the at least onepattern element of the set of pattern elements is asymmetric.
 17. Thelithography system of claim 14, wherein the one or more indicators ofthe location of the sample comprise: at least one of a position of theset of fabricated elements, a distance between two elements of the setof fabricated elements, or a sidewall angle of at least one of the setof fabricated elements.
 18. The lithography system of claim 14, whereinthe set of pattern elements comprises: a substantially opaque material.19. The lithography system of claim 17, wherein the substantially opaquematerial includes a metal.
 20. The lithography system of claim 14,wherein the set of pattern element comprises: at least one of a binarypattern element or a phase mask pattern element.
 21. A metrology system,comprising: a sample stage configured to support a substrate with ametrology target disposed upon the substrate, wherein the metrologytarget is associated with an image of a pattern mask generated by alithography system, wherein the pattern mask includes a set of patternelements configured to generate a set of diffracted beams includingillumination from an off-axis illumination pole of the lithographysystem, wherein at least two diffracted beams of the set of diffractedbeams received by the lithography system are asymmetrically distributedin a pupil plane of the lithography system, wherein the at least twodiffracted beams of the set of diffracted beams are asymmetricallyincident on the sample to form a set of fabricated elements of themetrology target, wherein the set of fabricated elements of themetrology target includes one or more indicators of a location of thesample along an optical axis of the set of projection optics of thelithography system; at least one illumination source configured toilluminate the metrology target; at least one detector configured toreceive illumination from the metrology target; and at least onecontroller communicatively coupled to the detector and configured todetermine the location of the sample along the optical axis of the setof projection optics based on the one or more indicators.
 22. Themetrology system of claim 21, wherein the illumination from themetrology target includes at least one of reflected illumination,scattered illumination, or emitted illumination.
 23. The metrologysystem of claim 21, wherein the pattern mask further comprises: anadditional set of pattern elements configured to generate an additionalset of diffracted beams including illumination from the illuminationpole of the lithography system, wherein at least two diffracted beams ofthe additional set of diffracted beams received by the set of projectionoptics are symmetrically distributed in the pupil plane of thelithography system, wherein the at least two diffracted beams of theadditional set of diffracted beams form an additional set of fabricatedfeatures of the metrology target corresponding to an image of theadditional set of pattern elements, wherein the additional set offabricated features are constant with respect to the location of thesample along the optical axis of the set of projection optics.
 24. Ametrology system, comprising: a sample stage configured to support asubstrate with a metrology target disposed upon the substrate, whereinthe metrology target is associated with an image of a pattern maskgenerated by a lithography system, wherein the pattern mask includes aset of pattern elements configured to generate a set of diffracted beamsincluding illumination from a first illumination pole and a secondillumination pole of the lithography system, wherein the first andsecond illumination poles of the lithography system are symmetricallydistributed with respect to an optical axis of the lithography system,wherein at least two diffracted beams of the first set of diffractedbeams received by the lithography system are symmetrically distributedin a pupil plane of the lithography system, wherein at least twodiffracted beams of the second set of diffracted beams received by theset of projection optics overlap the first set of diffracted beams inthe pupil plane of the lithography system, wherein the at least twodiffracted beams of the first and second sets of diffracted beams of theset of diffracted beams are symmetrically incident on the sample to forma set of fabricated elements of the metrology target, wherein the set offabricated elements of the metrology target includes one or moreindicators of a dose of illumination on the sample associated with atleast two diffracted beams of the first and second sets of diffractedbeams; at least one illumination source configured to illuminate themetrology target; at least one detector configured to receiveillumination from the metrology target; and at least one controllercommunicatively coupled to the detector and configured to determine thedose of illumination on the metrology target associated with the atleast two diffracted beams of the first and second sets of diffractedbeams based on the one or more indicators.
 25. The metrology system ofclaim 24, wherein the illumination from the metrology target includes atleast one of reflected illumination, scattered illumination, or emittedillumination.
 26. The metrology system of claim 24, wherein the patternmask further comprises: an additional set of pattern elements configuredto generate an additional set of diffracted beams including illuminationfrom the illumination pole of the lithography system, wherein at leasttwo diffracted beams of the additional set of diffracted beams receivedby the set of projection optics are symmetrically distributed in thepupil plane of the lithography system, wherein the at least twodiffracted beams of the additional set of diffracted beams form anadditional set of fabricated features of the metrology targetcorresponding to an image of the additional set of pattern elements,wherein the additional set of fabricated features are constant withrespect to the location of the sample along the optical axis of the setof projection optics.
 27. A method for determining a position of asample along an optical axis of a lithography system, comprising:generating an image of a pattern mask with a lithography systemincluding an off-axis illumination pole, wherein the pattern maskincludes a set of pattern elements configured to generate a set ofdiffracted beams including illumination from an off-axis illuminationpole of the lithography system, wherein at least two diffracted beams ofthe set of diffracted beams received by the lithography system areasymmetrically distributed in a pupil plane of the lithography system,wherein the at least two diffracted beams of the set of diffracted beamsare asymmetrically incident on the sample to form a set of fabricatedelements of the metrology target, wherein the set of fabricated elementsof the metrology target includes one or more indicators of a location ofthe sample along an optical axis of the set of projection optics of thelithography system; measuring the one or more indicators of the locationof the sample along the optical axis of the set of projection optics ofthe lithography system using a metrology system; and determining thelocation of the sample along the optical axis of the set of projectionoptics based on the one or more indicators.
 28. A method for determininga dose of illumination in a lithography system, comprising: generatingan image of a pattern mask with a lithography system including anoff-axis illumination pole, wherein the pattern mask includes a set ofpattern elements configured to generate a set of diffracted beamsincluding illumination from a first illumination pole and a secondillumination pole of the lithography system, wherein the first andsecond illumination poles of the lithography system are symmetricallydistributed with respect to an optical axis of the lithography system,wherein at least two diffracted beams of the first set of diffractedbeams received by the lithography system are symmetrically distributedin a pupil plane of the lithography system, wherein at least twodiffracted beams of the second set of diffracted beams received by theset of projection optics overlap the first set of diffracted beams inthe pupil plane of the lithography system, wherein the at least twodiffracted beams of the first and second sets of diffracted beams of theset of diffracted beams are symmetrically incident on the sample to forma set of fabricated elements of the metrology target, wherein the set offabricated elements of the metrology target includes one or moreindicators of a dose of illumination on the sample associated with atleast two diffracted beams of the first and second sets of diffractedbeams; measuring the one or more indicators of the dose of illuminationon the metrology target associated with at least two diffracted beams ofthe first and second sets of diffracted beams; and determining the doseof illumination on the metrology target based on the one or moreindicators.